Cell-mediated cytotoxicity to porcine aortic endothelial cells is not dependent on galactosyl residues when baboon peripheral blood lymphocytes are previously primed with pig xenoantigens

BACKGROUND: In the pig-to-baboon model, acute vascular rejection remains the main hurdle for successful long-term xenograft survival. The production of galactosyl knockout pigs could solve concomitantly the problem of hyperacute and acute vascular rejection. This work studies in vitro the cell-mediated cytotoxicity of natural killer (NK) and T cells after priming of baboon peripheral blood lymphocytes (PBLs) with pig antigens to evaluate whether cytotoxicity is galactosyl-dependent. MATERIAL AND METHODS: PBLs from naive and primed baboons were used as effectors on primary porcine aortic endothelial cells (PAECs) to assess cytotoxicity. Untreated or galactosidase-digested PAECs were used to evidence the role of galactosyl residues on cell-mediated cytotoxicity. Two rat-anti baboon monoclonal antibodies were tested to inhibit either T+NK cells (LO-CD2b) or NK cells alone (LO-CD94). RESULTS: When using PBLs from naive animals, spontaneous lysis occurred and was inhibited by both LOCD-2b and LO-CD94. In comparison, lysis of PAECs was significantly higher when baboon PBLs were first primed in vivo with pig xenoantigens. In this case, cytotoxicity was completely inhibited by LO-CD2b but only partially by LO-CD94. Reduction of galactosyl residues by galactosidase digestion showed that PAEC lysis almost completely disappeared with naive baboon PBLs but not with primed baboon PBLs, thereby indicating that anti-pig T-cell response is not dependent on galactosyl residues. CONCLUSION: Galactosyl knockout pigs could solve hyperacute rejection and also prevent the activation of NK cells even after xenogeneic priming. T cells will then be the next hurdle for the success of xenografting.     

Anti-tumor x anti-CD3 heteroconjugates direct human peripheral blood lymphocytes to lyse colon tumor cells

T lymphocytes normally recognize antigens through the antigen receptor complex (TCR/CD3) but can be redirected to bind and lyse unrecognized tumor cells by anti-tumor X anti-CD3 heteroconjugates. Chemical coupling of an antibody directed against the T cell receptor complex and an antibody directed against tumor antigen produces a conjugated antibody that activates the T cell lytic mechanism and bridges the T cell and tumor cell. We tested the lytic activity of heteroconjugate-treated cultured human peripheral blood lymphocytes (PBLs) in 4-h radioactive chromium release assays with human colon tumor cell targets. PBLs were enriched for T cells by the depletion of Leu11a+ and Leu19+ cells, prior to culture in rIL-2 and anti-CD3. Cultured human PBLs depleted of Leu11a+ and Leu19+ cells produced low levels of tumor cell lysis in the absence of antibodies. Anti-tumor X anti-CD3 heteroconjugates significantly enhanced tumor cell lysis by cultured PBLs when tested against four different colon tumor cell lines (P less than 0.005), but, heteroconjugates in the absence of PBLs did not augment tumor cell lysis. Cultured PBLs treated with monoclonal anti-tumor antibody, with monoclonal anti-CD3 antibody, or with irrelevant heteroconjugate did not enhance tumor cell lysis. We conclude that heteroconjugate-directed lysis is mediated by PBLs and that both the anti-tumor antibody and the anti-CD3 antibody are essential for heteroconjugate function. In addition, heteroconjugate-enhanced tumor cell lysis is mediated through a mechanism other than antibody-dependent cellular cytotoxicity or nonspecific T cell receptor crosslinking.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human anti-pig T-cell mediated cytotoxicity

Abstract: Miniature swine have a variety of advantages as potential donors for human xenotransplantation, including size, physiological similarities, and breeding characteristics. To investigate the nature of the human anti-pig xenogeneic cellular response, we performed standard ⁵¹Cr-release cell-mediated lympholysis (CML) experiments. The major histocompatibility complex (MHC) allele specificity of the xenogeneic cellular response was tested on porcine target cells of three distinct homozygous MHC haplotypes (SLA^aa, SLA^CC, SLA^dd) and three intra-MHC recombinant haplotypes (SLA^jj, SLA^gg, SLA^kk), obtained from our herd of partially inbred miniature swine. After stimulation with irradiated porcine peripheral blood mononuclear cells (PBMC) of SLA^aa haplotype, a strong nonspecific cytotoxic response of the bulk culture against xenogeneic targets of all three haplotypes was observed. However, SLA allele specificity could be demonstrated after T cell enrichment, and mapping experiments revealed predominantly SLA class I restriction of xenoreactive cytotoxic T lymphocytes (CTLs), although some class II restriction was also observed. The experiments were repeated in the presence of anti-T cell monoclonal antibodies, anti-CD3 (OKT3), anti-CD2 (35.1), anti-CD4 (OKT4), or anti-CD 8 (OKT8). The bulk xenogeneic CML was not inhibited by any of the anti-T cell antibodies tested. However, after T cell-enrichment, lysis of porcine targets was significantly inhibited by anti-CD3 or anti-CD8 antibody and partially inhibited by anti-CD2 antibody. In comparable assays, the human allogeneic CML was blocked by anti-CD3 and anti-CD8, but not by anti-CD2 or anti-CD4 antibodies. Finally, the cytotoxic activity of A3b3, a human CD4⁺ T-cell clone, was tested. A3b3 lysed xenogeneic targets of SLA^aa haplotype, but not SLA^CC or allogeneic targets, and was inhibited by anti-CD4, anti-CD2, and anti-CD3 antibodies, but not by anti-CD8. With the aid of intra-MHC recombinant haplotypes, the xenogeneic CML reactivity of A3b3 was mapped to SLA class II, suggesting direct xenogeneic recognition of porcine MHC class II antigens by human T cells. Thus, the human anti-pig cell-mediated cytotoxic response is similar in magnitude to comparable allogeneic responses, and involves both SLA class I and class II restricted T-cell mediated cytotoxicity, as well as additional nonspecific killing, possibly by NK cells.     

Generation of mouse cytolytic T cells against human concanavalin A blasts. Involvement of non-MHC determinants in the antihuman response

Normal human peripheral blood lymphocytes (PBL) are incapable of eliciting a significant murine cytotoxic T cell (CTL) response either in vivo or in vitro. However, using a primary in vivo and secondary in vitro stimulation with lectin-activated PBL, Thy-l-positive cytotoxic cells were produced. The antigens that these T-cells identified were independent of the serum source employed in the culture medium used for lectin activation. The cells always preferentially lysed cells from the immunizing individual but were also able to lyse target cells from unrelated individuals, regardless of HLA identity or disparity with the immunizing individual, suggesting the presence of both a private (possibly class II antigens) and public specificity. Using the lymphoblasts of different family members as immunogen and targets there was slight preference of the CTL for HLA-identical targets with no apparent difference between the lysis exhibited against semiidentical and nonidentical subjects. Monoclonal antibodies directed against HLA DR or beta 2-microglobulin failed to inhibit the cytotoxicity observed in these experiments. It is suggested that under these circumstances of xenogeneic education, non-MHC-restricted T cells may become cytotoxic, and this model may serve as a useful probe to investigate some of the less-well-defined aspects of the T cell repertoire.     

Flow cytometry complement-mediated cytotoxicity assay detects baboon xenoantibodies directed to porcine epitopes undetected by hemolytic assay

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. To obtain information about the nature of cytotoxic response in pig-to-baboon xenotransplants, we sought to determine if serum cytotoxicity in this model was assay dependent. Sera from nine kidney or heart xenotransplanted baboons were obtained before transplantation and at the time of acute humoral xenograft rejection (AHXR). Cytotoxicity was measured by an anti-pig haemolytic assay (APHA) and by a flow cytometry complement-dependent assay (FCCA), using pig blood lymphocytes (PBLs). Serum samples showing inter-assay differences were absorbed with pig erythrocytes and assayed by APHA and FCCA, as well as by measuring anti-alphaGal and total anti-pig xenoantibodies. The results showed that in four AHXR samples, FCCA cytotoxicity was higher than APHA cytotoxicity. Absorption with pig erythrocytes diminished FCCA and removed APHA cytotoxicity. Residual FCCA activity was due to total anti-pig and IgM anti-alphaGal and non-Gal antibodies. Our results indicate that some cytotoxic antibodies present in the sera of xenotransplanted baboons at time of AHXR are IgM antibodies directed against pig PBL antigens not detected by APHA.     

Induction of cytotoxicity from human lymphocytes coated with bispecific antibody against human glioma cells

A bifunctional hetero-F (ab') 2 fragment containing the Fab portions from anti-CD3 and anti-glioma monoclonal antibodies was prepared. The antibody simultaneously recognized two different molecules, the CD3 complex on effector T cells and human glioma-associated antigens on target glioma cells. This bispecific F (ab') 2 fragment induced peripheral blood lymphocytes (PBLs) from healthy donors to lyse cells of the human glioma cell line, U251MG, that is resistant to natural killer cell-mediated cytolysis. Compared with lymphokine-activated killer (LAK) activity which is obtained by exposure to interleukin (IL)-2 for more than 3 days, the maximum bispecific antibody-dependent cytotoxicity can be generated only after 24 hour exposure to IL-2. And cytotoxicity of lymphocytes triggered by the bispecific antibody was dependent upon the concentration of IL-2 in the culture medium. The effect of the bispecific antibody on LAK cells was tested in patients suffering from malignant glioma. One patient who received specific targeting therapy (LAK plus bispecific antibody) showed the disappearance of high density tumor mass from CT scan. But the patient who received only LAK therapy showed the recurrence of tumor one year after LAK treatment. These are preliminary data, but may be a promising approach in cancer immunotherapy.     

CD4+ cells play a major role in xenogeneic human anti-pig cytotoxicity through the Fas/Fas ligand lytic pathway

BACKGROUND: In this study, the role of cell-mediated cytotoxicity by human leukocytes against pig endothelial cells was examined in vitro. The aim was to determine which cell subsets were responsible for this phenomenon and which pathways were involved in cell lysis. METHODS: Primed human peripheral blood mononuclear cells (PBMC) or purified CD4+ or CD8+ T cells were used in a cell-mediated cytotoxicity assay in which cytotoxicity of an SV40 transformed porcine endothelial cell (EC) line (SVAP) was determined by Annexin V binding. RESULTS: Human PBMC demonstrated specific lysis of porcine EC that was proportional to the effector: target ratio. CD4+ T cells accounted for >60% of this lysis, whereas CD8+ T cells accounted for <20%. CD4+ T cell-mediated lysis depended on direct recognition of porcine major histocompatibility complex class II molecules as inhibition of swine leukocyte antigen class II on porcine EC-inhibited CD4+ T cell cytotoxicity. This lysis was mediated through the Fas/FasL pathway as addition of anti-Fas and/or anti-FasL antibody profoundly inhibited antiporcine lysis. In addition, FasL gene expression was detected in primed PBMC and CD4+ T cells by RT-PCR, whereas granzyme B gene expression was not. Primed CD4+ T cells demonstrated high level FasL protein by Western blotting and two-color FACS analysis, whereas NK cells and CD8+ T cells did not. Finally, recombinant human FasL induced apoptosis in Fas expressing porcine EC cells, demonstrating that human FasL interacted with and activated Fas on porcine EC cells. CONCLUSIONS: In conclusion, human to pig cell-mediated cytotoxicity was mediated predominantly by CD4+ T cells through the Fas/FasL pathway of apoptosis. These results suggest that direct cytotoxicity by xenoreactive CD4+ T cells may be one of several effector mechanisms involved in cellular xenograft rejection.     

Target cell membrane modification and susceptibility to lymphokine activated killer cell mediated lysis. I. Trinitrophenyl has no effect on murine normal cells and tumors

Lymphokine activated killer (LAK) cells selectively recognize and lyse a wide variety of tumor cells, including syngeneic, allogeneic, and xenogeneic cells, by a mechanism which remains unknown. It has been shown that trinitrophenyl modification of human peripheral blood lymphocytes (PBLs) and murine blasts induces sensitivity to lysis by LAK cells. We undertook these studies to examine the effects of trinitrophenyl (TNP) on murine normal cells and tumors. Fresh murine PBLs are resistant to LAK mediated lysis and TNP modification of the cell surface has no effect, in direct contrast to results with human cells. However, the addition of anti-TNP antibody demonstrated lysis of the TNP modified cells by an antibody dependent cellular toxicity mechanism showing that these cells can be lysed by LAK cells. Furthermore, TNP did not induce sensitivity to LAK cell mediated lysis on other normal murine tissues including kidney and bone marrow. The lysis of YAC tumor cells is not inhibited by the addition of TNP modified PBLs. A single cell assay demonstrated no conjugation of LAK effector cells and TNP modified PBLs. LAK cells derived from murine PBLs, as is done in the human, were unable to lyse the TNP modified PBLs, ruling out the origin of the LAK cells as the reason for the different results in humans and mice. These observations represent a difference in the spectrum of target cells lysed by human and murine LAK cells, and also demonstrate a response to TNP modification by fresh murine normal tissues and tumors that differs from that observed with murine blasts.(ABSTRACT TRUNCATED AT 250 WORDS)     

Xenogeneic human anti-pig cytotoxicity mediated by activated natural killer cells

Abstract: Even if hyperacute rejection, which is mediated by human natural antibodies (nAb) and complement, could be prevented, xenoreactive human anti-pig cellular responses may lead to delayed and/or chronic xenograft rejection. Among the cell populations participating in such rejection, NK cells have been proposed as an important component. In this study we report the in vitro cytotoxic activity of natural killer (NK) cells obtained from healthy human donors against porcine target cells. Freshly isolated peripheral blood mononuclear cells (PBMC) and purified NK cells (CD16+/CD56+, CD3-, CD20-, CD33-) exhibited little or no cytotoxic activity when tested on porcine phytohemagglutinin (PHA)-stimulated lymphoblasts or bone marrow- or aortic-derived endothelial cell lines in the presence of serum-free medium. Killing was considerably higher in the presence of human decomplemented plasma, containing xenoreactive nAb, or purified Gal(α1,3)Gal-reactive antibodies, suggesting that antibody dependent cell-mediated cytotoxicity (ADCC) mediated by NK cells is an important mechanism involved in xenogeneic cytotoxicity. After incubation of human PBMC for 6 days in the presence of irradiated xenogeneic porcine or allogeneic stimulator cells, or in the presence of exogenous interleukin 2 (IL-2), the cytotoxic activity of the bulk cultures as well as that of isolated NK cells (separated from stimulated bulk cultures) against xenogeneic targets increased considerably, and corresponded to an increased NK-specific lysis of K562 target cells. Cell surface staining and flow cytometry showed that CD16+/CD56+, CD3- NK cells composed ca. 25% of short-term (6 days) xenogeneic, allogeneic, or IL-2 stimulated bulk cultures. In summary, these data suggest that, in contrast to allogeneic cell- mediated killing, xenogeneic human anti-porcine cytotoxicity includes an important contribution from NK cells.     

Renal allograft rejection. Possible involvement of antibody-dependent cell-mediated cytotoxicity

We have demonstrated that serum from appropriately sensitized patients can contain IgG antibodies that bind to cultured renal epithelial cells. The presence of such antibodies on the surface of renal cells enables otherwise nonlytic PBMC to lyse these renal cells by an antibody-dependent cell-mediated cytotoxicity (ADCC) mechanism. Experiments involving cell-sorting and specific complement-mediated lysis showed that the ADCC effector cells were of the CD3 -ve, C16 +ve phenotype characteristic of NK cells. In this report it is argued that an ADCC mechanism may be of importance in mediating chronic renal cell damage in the absence of acute allograft rejection.     

Individual human serum differs in the amount of antibodies with affinity for pig fetal ventral mesencephalic cells and the ability to lyse these cells by complement activation

Xenografting pig fetal ventral mesencephalic (pfVM) cells to repair the dopamine deficit in patients with Parkinson's disease is the focus of both experimental and clinical investigations. Although there have been marked advances in the experimental and even clinical application of these xenogeneic transplantations, questions regarding the host's xenospecific immune response remain unanswered. It has been shown that human serum is able to lyse pfVM tissue by both anti-gal-gal and non-anti-gal-gal antibodies by complement activation. The aim of this study was to investigate whether interindividual differences exist in the levels of pfVM cell-specific IgM and IgG subclass antibodies, their ability to lyse pfVM cells in vitro and the relationship between both. Pig fetal VM cells were incubated with heat-inactivated serum from 10 different individuals and binding of IgM antibodies and IgG subclass antibodies to pfVM cells was analyzed by flow cytometry. The ability to lyse pfVM cells was analyzed exposing 51Cr-labeled pfVM cells to fresh serum or isolated IgM and IgG from the same individuals and subsequent determination of released 51Cr from lysed cells. Strong differences were found between individuals in the levels of pfVM cell-specific IgM antibodies: antibody levels differed up to 40-fold. pfVM-specific IgG1 and IgG2 levels were only detectable in a few individuals. The ability to lyse pfVM cells ranged from negligible lysis up to 66.5% specific lysis. There was a strong correlation between the levels of individual pfVM-specific IgM antibodies and the ability to lyse pfVM cells in vitro. Isolated IgM, but not IgG, was able to lyse pfVM cells in the presence of complement. In conclusion, the interindividual differences in the levels of IgM with affinity for pfVM cells and their ability to lyse pfVM cells in vitro are considerable. Only few individuals possessed IgG1 and IgG2 subclass antibodies with affinity for pfVM. These findings may influence patient selection for porcine transplants and chances of graft survival in individual patients.     

The molecular mechanism of apoptosis induced by xenogeneic cytotoxicity

Abstract: In order to clarify the role of natural killer (NK) cells in delayed xenograft rejection (DXR) of discordant xenotransplantation, we used in vitro xenogeneic combination of human NK cells and pig kidney target cells (PK13, and investigated the mechanism of xenogeneic cytotoxicity caused by human NK cells. In the presence of decomplemented human serum or human IgG, freshly isolated human peripheral blood lymphocytes (PBLs) caused both membrane (⁵¹Cr release) and DNA (³H release) damage on PK15. In contrast, only membrane damage was detected in the presence of normal human serum. To clarify the participation of perforin/granzymes-cell mediated cytotoxicity (P/G-CMC), when EGTA or concanamycin B (CMB) was added to the cytotoxicity assays, both cytotoxicities were completely inhibited by these drugs in a dose-dependent manner. In terms of the involvement of Fas/FasL-based cytotoxicity (F-CMC), while the cytotoxicity assays were performed in the presence of antagonistic anti-human FasL mAb, this antibody was not able to block the cytotoxicity. From these results, it is concluded that xenogeneic cytotoxicity is due to NK cell dependent ADCC (antibody-dependent cell-mediated cytotoxicity), and their effector mechanism can cause apoptosis on target cells via P/G-CMC.     

Prevention of graft-versus-host disease using antibody to Thy 1. A role for complement in vivo

An in vitro assay was used to determine whether mouse complement (C') was able to lyse T cells coated with a monoclonal IgM anti-Thy-1 antibody. Using lymph node cells (LNC) as targets, it was found that antibody-coated mature T cells were efficiently lysed by serum from C57BL/6, but inefficiently lysed by serum from BALB/c mice. Serum from both strains lysed antibody-coated thymocytes, demonstrating that BALB/c serum is not deficient in one of the C' components. The lytic activity of BALB/c serum could be improved by increasing the concentration of monoclonal anti-Thy-1 used to coat the target LNC. In contrast, serum from C5-deficient A/HeJ mice was unable to lyse either of the antibody-coated target cells in vitro. These in vitro results were confirmed in vivo using a model of lethal graft-versus-host disease (GVHD) induced to minor histocompatibility antigens. In this model, GVHD occurs when lethally irradiated mice are transplanted with bone marrow plus spleen cells from H-2-identical donor mice. GVHD is prevented when T cells are removed from the cells by in vitro treatment with anti-Thy-1 antibody plus rabbit C'. As predicted by the in vitro data, C57BL/6 mice were protected from GVHD when transplanted with LP cells treated with a 1:100 dilution of anti-Thy-1 antibody, but without complement, but BALB/c recipients of antibody-coated B10.D2/nSN cells were not. When the concentration of anti-Thy-1 used to coat the donor cells was increased to 1:50, 8 of 9 recipients were protected from GVHD. In contrast, none of the C5-deficient DBA/2 recipients of anti-Thy-1 (1:50 dilution) treated B10.D2 cells were protected from GVHD. Complement-mediated lysis appears to be the primary mechanism by which T cells coated with a monoclonal IgM anti-Thy-1 antibody are functionally inactivated in vivo.     

Human IgM xenoreactive natural antibodies can induce resistance of porcine endothelial cells to complement-mediated injury

Abstract: It is thought that human IgM xenoreactive natural antibodies (nAbs) can induce activation of porcine endothelial cells independent of complement. Therefore we hypothesized that pretreatment of porcine endothelial cells with anti-pig nAbs may affect the ability of the endothelial cells, when subsequently incubated with a source of nAbs and complement, to bind antibodies and complement components and to undergo complement-mediated cytotoxicity. We preincubated porcine endothelial cells at 37°C for 1 hr or 40 hr with a source of nAbs. We then incubated these pretreated endothelial cells with a complement source that contained a normal complement level and a low level of IgM nAbs for 1 hr to measure bound IgM and IgG and complement components, and for 4 hr to measure cytotoxicity. We found that preincubation for as long as 40 hr did not impair the binding of IgM and IgG, implying no antibody-induced loss of membrane antigens from the endothelial cells, or the binding of C3bi, C4d, C6, and C9 upon complement activation. In contrast, preincubation for 40 hr with a nAb source induced in the endothelial cells marked resistance to complement-mediated killing. Resistance could be induced with purified human IgM but not with purified IgG or IgM-depleted human serum. The ability of purified IgM to induce resistance was abrogated by removal of anti-pig xenoreactive nAbs by absorption with pig endothelial cells, and induction of resistance required protein synthesis. We conclude that prolonged incubation of human anti-pig nAbs with pig endothelial cells does not cause loss of endothelial cell membrane antigens or impairment in binding of nAbs or complement components; instead, it induces marked resistance to complement-mediated cytotoxicity. These observations may be of value to develop strategies that enhance survival of a xenograft.     

Resistance and susceptibility of neural cells to lysis by cytotoxic lymphocytes and by cytolytic granules.

The susceptibility of neural cells to immune-mediated lysis by alloantigen-specific cytotoxic lymphocytes is important in understanding cell-mediated immune responses during rejection of transplanted neural tissues and in inflammatory responses of the central (CNS) and peripheral (PNS) nervous systems. In this study, we used 51Cr-release and granzyme A assays to examine whether primary cultures of astrocytes and CNS and PNS neurons could serve as targets for alloantigen-specific CTLs and granule-mediated lysis. The level of astrocyte killing by alloantigen-specific CTLs correlated with expression of the class I gene products of the major histocompatibility complex. Astrocytes cultured for 1-2 weeks did not express class I MHC antigens and were not susceptible to lysis by activated alloantigen-specific CTLs. Lengthening the astrocyte culture period to 3 weeks resulted in class I MHC antigen expression on the astrocyte surface and alloantigen-specific lysis. Astrocytes of all ages tested were susceptible to lysis by isolated cytolytic lymphocyte granules. PNS neurons of various ages tested also served as targets for CTLs and were lysed by isolated granules. In contrast, CNS neurons did not express class I MHC antigens and were highly resistant to killing by CTLs and lymphocyte granules. CNS neurons and astrocytes did not trigger specific granzyme A secretion from effector cells. In the presence of leucoagglutinin, CTLs-specific recognition of target cells is bypassed, and virtually any cell, regardless of its antigens, is killed nonspecifically. Although leucoagglutinin-treated CNS neurons and astrocytes triggered increased granzyme A secretion from effector cells, only astrocytes were lysed in an antigen-nonspecific manner, whereas CNS neurons remained strikingly resistant. These results suggest differences in the susceptibility of PNS and CNS neurons to T cell-mediated lysis. CNS neurons appear to possess protective mechanisms that render them refractory to CTL-mediated lysis and granule-mediated lysis, whereas PNS neurons and astrocytes are far more susceptible to both types of immune attack.     

Regulation of cytotoxic T lymphocyte-mediated graft rejection following bone marrow transplantation

Cytotoxic T lymphocytes have been implicated as the effector cell mediating graft rejection following human allogeneic bone marrow transplantation. We have studied a BMT patient who rejected haploidentical T cell-depleted marrow. In vitro studies demonstrated that the circulating lymphocytes were CD3+ and CD8+, of recipient origin, and exhibited selective cytotoxicity against donor-specific class I major histocompatibility complex antigens. Cytotoxicity was inhibited by monoclonal antibodies directed against CD3, CD8, CD2, and lymphocyte function-associated antigen-1 on the T cell, and against MHC class I proteins on the target cell. Furthermore, these circulating cells inhibited the in vitro growth and differentiation of enriched donor bone marrow progenitor cells, an inhibition that was partially reversed by anti-CD3 MAb. Donor-specific recipient-derived CTL may mediate resistance to engraftment, and CTL activity may be inhibited by a number of MAb. The implications of these findings for host preparation and treatment are discussed.     

Neonatal porcine islet cells induce human CD4+, but not CD8+, lymphocyte proliferation and resist cell-mediated cytolytic injury in vitro.

Xenotransplantation of porcine tissue to human recipients promises to alleviate the organ shortage. Human antibody-mediated and cell-mediated immune responses against porcine grafts, however, represent barriers to successful xenotransplantation. We compared neonatal porcine islet cells (NPICs) and neonatal porcine splenocytes for the ability to stimulate proliferation of human peripheral blood lymphocytes (PBLs), and for their susceptibility to human natural killer (NK) and cytotoxic T-lymphocyte (CTL)-mediated lysis. Human peripheral blood CD4+ lymphocytes showed strong proliferation in response to NPICs, likely because of occasional swine leukocyte antigen (SLA) class II+ cells in the NPIC preparations. In contrast, human peripheral blood CD8+ lymphocytes did not proliferate in response to NPICs, although they showed clear responses to both porcine splenocytes and endothelial cells. Both human CTL-raised-against-porcine splenocytes and endogenous NK cells lysed porcine splenocytes, but the same cells showed little or no lytic activity against NPICs. Lysis of porcine splenocyte targets was completely abrogated by pretreatment of the human NK or CTL populations with concana-mycin A, suggesting a perforin-dependent effector mechanism. Pretreatment of the NPIC targets with proinflammatory porcine cytokines to upregulate SLA class I expression failed to enhance human CTL-mediated lysis. However, lysis of NPICs by human CTLs could be elicited when a lectin was added to form stable effector:target cell conjugates. It appears that NPICs do not express sufficiently high levels of co-stimulatory and/or adhesion molecules to either activate human CD8+ T-cells or to be effective targets for activated human CTLs. These data suggest that NPICs may not be destroyed by NK- or CTL-mediated lytic mechanisms after transplantation into humans.     

The role of LFA-1 in the lysis of bladder cancer cells by bacillus Calmette-Guérin and interleukin 2-activated killer cells

Activated cytotoxic effector cells such as bacillus Calmette-Guérin (BCG)-activated killer (BAK) and lymphokine-activated killer (LAK) cells are thought to mediate the antitumor effects in the immunotherapy of superficial bladder cancer with BCG. We investigated the role of the leukocyte-function-antigen-1 and its two subunits CD11a and CD18 in the lysis of bladder tumor cells by both effector cell populations. We used flow cytometry to measure CD11a and CD18 expression on BAK and LAK cells. The involvement of both surface molecules in the lysis of bladder tumor cells was determined by phase contrast microscopy and a set of radioactive-release assays using specific inhibitory antibodies. BAK and LAK cells expressed more CD11a and CD18 on their surfaces than unstimulated peripheral blood mononuclear cells. Effector-target cell adhesion was a prerequisite for the cell-mediated cytotoxicity of BAK and LAK cells against bladder tumor targets. Cytotoxicity of both BAK and LAK cells was inhibited by a combination of anti-CD11a and -CD18 monoclonal antibodies. Our study gives further insight into the complex interactions of the adhesion molecules of activated immune cells and their respective tumor targets and might help to increase our knowledge of the molecular mechanisms of BCG-immunotherapy.     

Variability of anti-αGal antibodies in human serum and their relation to serum cytotoxicity against pig cells

Abstract: One of the major obstacles in pig-to-human xenografting is hyperacute rejection (HAR) of pig cells caused by preformed anti-pig antibodies and complement. In 1991 we suggested that anti-αGal antibodies play a major role in the HAR of pig cells. Anti-αGal antibodies recognize terminal α-galactose-containing epitopes on glycoproteins and glycolipids. They are present in humans, apes, and Old World monkeys, but not in lower mammals such as pigs. However, pigs, unlike humans, express terminal α-galactose epitopes on vascular endothelium which represent targets for human anti-αGal antibodies. Despite increasing recognition that anti-αGal antibodies are an important factor, many questions related to their precise role in HAR remain to be answered.
In this study, we analyzed 75 human AB sera in terms of (i) cytotoxic activity against cultured pig (PK-15) cells, (ii) anti-αGal titers, (iii) immunoglobulin-binding to pig cells, and (iv) immunoglobulin concentrations. The results demonstrated considerable variability in cytotoxicity, anti-αGal titers, and immunoglobulin binding to pig cells, whereas the serum immunoglobulin concentrations were less variable. Positive correlations were found between cytotoxicity and binding of IgG and IgM to the surface of pig cells. The surface-bound IgG and IgM also correlated with the serum anti-αGal IgG and IgM titers. Anti-αGal IgA, however, did not show any relation with cytotoxicity or cell binding. Concentrations of serum total immunoglobulins correlated with neither I cytotoxic activity nor cell binding.     

Fine specificity of xenogeneic antigen recognition by human T cells

T cell mediated responses play a role in xenograft as well as allograft rejection. In vitro T cell responses to xenogeneic antigens are characterized by T cell recognition of polymorphic determinants of gene products encoded by the major histocompatibility complex of the stimulating cells, but little is known of the fine specificity of this recognition of xenogeneic antigens and its comparability to allogeneic antigen recognition. In order to study the fine specificity of human CTL in the recognition of xenogeneic antigens, long-term lines or clones were established from a secondary mixed lymphocyte response in which the stimulator cells were H-2b murine splenocytes. By comparing the ability of a series of target cells derived from congenic recombinant mouse strains to be lysed by these CTL, it was demonstrated that all isolated lines specifically lysed target cells expressing H-2b or T1a/Qa-1b products. Of the effector populations specific for H-2b cell surface molecules, all recognized class I products with Kb specificity predominating when evaluated for their ability to lyse in vivo-derived class I mutants or cells transfected with class I genes. These human xenoreactive CTL were able to distinguish wild type Kb molecules from those altered by amino acid changes confined to a single molecular domain of Kb. These findings demonstrate that xenogeneic antigen recognition by human T cells is characterized by a fine specificity of antigen recognition comparable to the specificity of recognition of major histocompatibility complex-encoded molecules by murine CTL across allogeneic differences.