High frequency of malaria-specific T cells in non-exposed humans.

A major goal of current candidate malaria vaccines is to stimulate the expansion of clones of malaria-specific lymphocytes. We have examined the in vitro T cell responses of a group of malaria exposed and non-exposed adult Caucasian donors to recombinant circumsporozoite (CS) proteins, one of which is undergoing clinical trials, to blood-stage parasites, and to synthetic peptides copying the CS protein and defined blood-stage proteins. In nearly all individuals tested, CD4 T cell proliferation or lymphokine production occurred in response to whole parasite or CS protein stimulation, and T cells from many individuals responded to synthetic peptides. T cell responses were major histocompatibility complex-restricted, and stimulation of T cells with malaria parasites or CS protein did not appear to expand a population of T cell receptor gamma/delta cells. Malaria-specific responses were independent of prior malaria exposure, and in some cases exceeded the magnitude of response to tetanus toxoid. Specific T cells are present in high frequency in the peripheral blood of many donors who have never been exposed to malaria. Although malaria-specific CD4 T cells play an important role in immunity, these data question whether vaccines need to stimulate such cells, and focus attention on other aspects of malaria immunity which may be more critical to a successful vaccine.     

Polyclonal in vitro proliferative responses from nonimmune donors to Plasmodium falciparum malaria antigens require UCHL1+ (memory) T cells.

The in vitro polyclonal proliferative responses of peripheral blood mononuclear cells to whole blood stage parasites or fractionated antigens from the human malaria parasite Plasmodium falciparum were studied. Cells from healthy laboratory donors who had never been exposed to malaria antigens in vivo consistently proliferated to P. falciparum antigens, as did cord blood mononuclear cells. This response was only observed in sheep rosette-positive cells in the presence of adherent cells and was inhibited by NH4Cl, indicating a requirement for antigen processing. The proliferative response was strongest at day 6 and was dependent on the presence of cells expressing high levels of CD45 180-kD isomer (UCHL1 monoclonal antibody), a marker for activated or memory cells, but not for CD45R (SN130 monoclonal antibody) a marker for naive or unprimed T cells. This suggests a similarity to the recall response to tuberculin antigen. These results suggest that the proliferative response to malaria antigens observed previously and described as a nonspecific mitogenic response may be a cross-reactive response to epitopes shared between P. falciparum and other common immunogens. This would explain the establishment of T cell clones to malaria antigens from such donors, but might suggest that the epitopes to which such clones are specific may be of questionable protective or diagnostic use.     

Naive human alpha beta T cells respond to membrane-associated components of malaria-infected erythrocytes by proliferation and production of interferon-gamma.

Crude extracts of Plasmodium falciparum schizont-infected erythrocytes (PfSE) induce polyclonal activation of peripheral blood T lymphocytes from naive (malaria unexposed) humans. We demonstrate that the active component of PfSE is membrane bound, soluble in sodium dodecyl sulphate (SDS) and partially heat stable, but distinct from the tumour necrosis factor (TNF)-inducing, exoantigen-like activity of schizont extracts. Malaria pigment induces little or no T-cell activation. The responding cells are predominately CD4+, CD45RO+, T-cell receptor (TCR) alpha beta+. Contrary to previous reports, expansion of the TCR gamma delta+ subset was observed in cells from only one of eight donors. Proliferating cells secrete interferon-gamma (IFN-gamma) and release large amounts of soluble interleukin-2R (sIL-2R) into the culture supernatant but produce no detectable interleukin-4 (IL-4), a phenotype typical of the T-helper (Th)1 subset of CD4+ T cells. We propose that these activated T cells may initiate the inflammatory response to malaria infection in non-immunes and may contribute to the pathology of the disease.     

Phenotypical and Functional Characterization of Double-Negative (CD4-CD8-) αβ T-Cell Receptor Positive Cells from an Immunodeficient Patient

We have characterized CD4-CD8- double-negative (DN) alpha beta TCR+ T cells from a patient with immunodeficiency, lymphocytosis, lymphadenopathy, and hepatosplenomegaly. The majority of peripheral blood lymphocytes were DN alpha beta TCR+ T cells as evaluated by FACS and biochemical analysis. The DN T cells showed the following phenotype: alpha beta TCR+, gamma delta TCR-, CD2+, CD3+, CD4-, CD5+, CD7-, CD8-, CD16-, CD25-, CD26-, CD28+, CD45RO-, CD45RA+, CD57+, and HLA-DR+. Both southern blot analysis of TCR genes and FACS analysis applying a panel of V beta and V alpha monoclonal antibodies (MoAbs) indicated a polyclonal T-cell expansion. Thymic biopsy showed normal histology, whereas lymph node biopsy samples showed altered histological and immunohistological patterns with markedly expanded paracortical areas containing the DN T cells of the same phenotype as found in peripheral blood T cells. In functional studies, the DN T cells showed a profoundly reduced proliferative response upon stimulation with mitogens as well as MoAbs against the TCR/CD3 complex, CD2, and CD28, respectively. Addition of exogenous interleukin-2 (IL-2) only minimally augmented the proliferative response. In contrast, the addition of a combination of Ca2+ ionophore and phorbol 12-myristate 13-acetate (PMA) restored the proliferative response of the DN T cells to almost normal levels. This observation strongly suggests that the protein kinase C activity of the DN T cells was intact, but that the normal mechanism for transmembrane signal transduction was impaired in these unusual DN T cells.     

Pathogenic autoantibody-inducing gamma/delta T helper cells from patients with lupus nephritis express unusual T cell receptors.

In previous work, we found that only 59 (15%) of 396 "autoreactive" T cell clones derived from five patients with lupus nephritis had the ability to selectively augment the production of pathogenic anti-DNA autoantibodies and the majority (49/59) of those autoimmune T helper (Th) clones were CD4+. Surprisingly, 7 of those Th clones were CD4-/CD8- and gamma/delta TCR+, capable of augmenting the production of pathogenic anti-DNA autoantibodies up to 125-fold. The gamma/delta Th clones responded in a MHC-nonrestricted manner to some endogenous autoantigen associated with heat shock proteins (HSP60) on the lupus B cells. The gamma/delta TCR genes expressed by 4 of these Th clones were amplified and sequenced here. Three of the 4 Th clones, each from a different lupus patient, expressed a gene from the V gamma 1 subgroup. Moreover, 2 of the Th clones expressed V delta 5, and the others V delta 1 or V delta 3. These TCRs are rarely expressed by peripheral blood gamma/delta T cells of normal adult humans. The predominant gamma/delta T cells in human peripheral blood express V gamma 2 (V gamma 9) and V delta 2 TCR genes, including HSP-responsive T cells. None of the lupus Th clones expressed this combination of TCR genes. In addition, some of these pathogenic autoantibody-inducing Th clones from the lupus patients had limited diversity and few N-nucleotide additions in their gamma/delta TCR junctional regions (CDR3), thus resembling fetal gamma/delta thymocytes early in ontogeny.     

Human peripheral blood gamma delta T cells respond to antigens of Plasmodium falciparum.

Peripheral blood lymphocytes from donors previously unexposed to malaria parasites proliferate in vitro when stimulated with whole parasitized red blood cells of several different strains of Plasmodium falciparum. Here we show that both cells enriched for both memory (CD45R0+) and na?ve (CD45R0-) phenotype can respond. Cells involved in these responses occur at frequencies similar to those observed for recall antigens such as tetanus toxoid but at lower frequencies than observed for the superantigens staphylococcal enterotoxin B or the mitogenic lectin phytohemagglutinin (PHA). Proliferation is inhibited by antibodies to class II MHC and to CD3 molecules. Stimulation of purified CD45R0- T cells by whole parasitized red blood cells for 6 days results in the generation of a large proportion of gamma delta T cell blasts of V gamma 9V delta 2 TCR phenotype and in the acquisition of the CD45R0 molecule within the blast cell population. The rapid generation of a vigorous primary in vitro gamma delta T cell response by malarial parasites may reflect the situation during primary malarial infection.     

IL-10 enhances expression of the IL-2 receptor alpha chain on T cells.

Interleukin-10 (IL-10) has various immunomodulatory actions depending on the target cell type. Some of these effects have been shown to be owing to its ability to down-regulate surface expression of markers, for example HLA-DR on macrophages and CD25 (IL-2 receptor alpha chain) on B cells. In this report we show that preincubation of IL-10 for 24 hr up-regulates expression of the activation marker CD25, but not HLA-DR on cloned T cells of various phenotypes such as CD4+, CD8+, CD4- CD8- alpha beta and gamma delta T-cell receptor (TCR)-expressing cells. This up-regulation of CD25 was accompanied by an increase in the T cells IL-2-dependent proliferative response in 63% of the CD4+ clones and 100% of the CD8+, CD4-, CD8- alpha beta and gamma delta TCR+ clones analysed. IL-10 was also shown to be at least partly responsible for the up-regulation of CD25 on mitogen-activated peripheral blood mononuclear cells, suggesting that IL-10 has this CD25 modulatory effect within a more physiological environment. Our data suggest that IL-10 can have a multitude of effects on human T cells, and should not be considered exclusively as an immunoinhibitory cytokine.     

Characterization of peripheral blood lymphocyte subsets in patients with acute Plasmodium falciparum and P. vivax malaria infections at Wonji Sugar Estate, Ethiopia

We investigated the absolute counts of CD4+, CD8+, B, NK, and CD3+ cells and total lymphocytes in patients with acute Plasmodium falciparum and Plasmodium vivax malaria. Three-color flow cytometry was used for enumerating the immune cells. After slide smears were stained with 3% Giemsa stain, parasite species were detected using light microscopy. Data were analyzed using STATA and SPSS software. A total of 204 adults of both sexes (age, >15 years) were included in the study. One hundred fifty-eight were acute malaria patients, of whom 79 (50%) were infected with P. falciparum, 76 (48.1%) were infected with P. vivax, and 3 (1.9%) were infected with both malaria parasites. The remaining 46 subjects were healthy controls. The leukocyte count in P. falciparum patients was lower than that in controls (P=0.015). Absolute counts of CD4+, CD8+, B, and CD3+ cells and total lymphocytes were decreased very significantly during both P. falciparum (P<0.0001) and P. vivax (P<0.0001) infections. However, the NK cell count was an exception in that it was not affected by either P. falciparum or P. vivax malaria. No difference was found in the percentages of CD4, CD8, and CD3 cells in P. falciparum or P. vivax patients compared to controls. In summary, acute malaria infection causes a depletion of lymphocyte populations in the peripheral blood. Thus, special steps should be taken in dealing with malaria patients, including enumeration of peripheral lymphocyte cells for diagnostic purposes and research on peripheral blood to evaluate the immune status of patients.     

Clonally expanded CD3+, CD4-, CD8- cells bearing the alpha/beta or the gamma/delta T-cell receptor in patients with the lymphoproliferative disease of granular lymphocytes.

Among 60 retrospectively assessed patients with the lymphoproliferative disease of granular lymphocytes (LDGL), lymphocytes from only 2 patients had the CD3+, CD4-, CD8- phenotype, rarely observed in normal peripheral blood lymphocytes (about 3%). In this paper we report a detailed study of lymphocytes isolated from these two patients. The cells from patients 1 had the CD3+, CD4-, CD8-, WT31-, beta F1-, TCR delta 1+, Ti gamma A-, BB3+, CD7+, CD16-, CD57+ phenotype, while cells from patient 2 had a phenotype even more rarely observed on normal lymphocytes: CD3+, CD4-, CD8-, WT31+, beta F1+, TCR delta 1-, CD7+, CD16-, CD57+. Thus, in only the first case the cells expressed the gamma/delta T-cell receptor (TCR) on the membrane, while the cells from the second case had the alpha/beta TCR. Genetic studies showed that in case 1 the TCR gamma gene was rearranged and the beta chain gene configuration was germline; the TCR mRNA was of normal size for the gamma chain, while that of the beta chain was truncated. Case 2 had the beta and the gamma genes of the TCR rearranged, but only the alpha and beta mRNA were expressed. In agreement with these findings, the delta chain gene of the TCR was rearranged in case 1 and was deleted in case 2. Cytotoxic activity was absent in cells from case 1 and low in case 2; in the latter, the lytic activity could be up-regulated following incubation with IL-2 or an anti-CD3 monoclonal antibody. Our study indicates that CD3+, CD4-, CD8- lymphocytes are rarely expanded in patients with LDGL. The detection of a lymphoproliferative disease of a CD3+, CD4-, CD8-, alpha/beta + cell may contribute to a better characterization of this novel lymphocytic subpopulation.     

Heterogeneity of lymphokine-activated killer (LAK) populations at the clonal level: both NK and CD3+, CD4-, CD8- clones efficiently mediate tumor cell killing

To investigate at the clonal level the phenotypic and functional properties of interleukin 2 (IL-2) activated killer cells (LAK), recombinant IL-2 activated peripheral blood lymphocytes were cultured under limiting conditions. Among 56 clones that lysed P815 in the presence of phytohemagglutinin (PHA) (22% of total proliferating microcultures) 36 clones lysed also the natural killer (NK)-sensitive K562 and the NK-resistant Hu126 glioma cell lines and one clone lysed only the K562 cell line. Several LAK clones were further assayed for both phenotype and functional activity. Of 22 clones, 10 were CD3-, CD4-, CD8-, and expressed the CD16 marker of NK cells; only one clone had the conventional phenotype of cytolytic T cells (CD3+, CD4-, CD8+), while 11 clones were CD3+, CD4-, CD8- and did not express alpha/beta heterodimer of T-cell antigen receptor as identified by WT31 monoclonal antibody. Only one of the latter clones was CD16+. Endogenous production of IL-2 after stimulation with PHA and phorbol myristate acetate was positive in 3/9 CD3- and in 8/8 CD3+, CD4-, CD8- clones. CD3- mediated strong antibody-dependent cellular cytotoxicity, a function exerted also by some CD3+, CD4-, CD8- T-cell clones to a lower extent. CD3+, CD4-, CD8- T-cell clones lysed different major histocompatibility complex unrelated tumor targets; moreover, this lytic activity seems to be CD3 dependent.     

The frequency of BDCA3-positive dendritic cells is increased in the peripheral circulation of Kenyan children with severe malaria

The ability of Plasmodium falciparum-infected erythrocytes to adhere to host endothelial cells via receptor molecules such as ICAM-1 and CD36 is considered a hallmark for the development of severe malaria syndromes. These molecules are also expressed on leukocytes such as dendritic cells. Dendritic cells are antigen-presenting cells that are crucial for the initiation of adaptive immune responses. In many human diseases, their frequency and function is perturbed. We analyzed the frequency of peripheral blood dendritic cell subsets and the plasma concentrations of interleukin-10 (IL-10) and IL-12 in Kenyan children with severe malaria and during convalescence and related these parameters to the adhesion phenotype of the acute parasite isolates. The frequency of CD1c(+) dendritic cells in children with acute malaria was comparable to that in healthy controls, but the frequency of BDCA3(+) dendritic cells was significantly increased. Analysis of the adhesion phenotypes of parasite isolates revealed that adhesion to ICAM-1 was associated with the frequency of peripheral blood CD1c(+) dendritic cells, whereas the adhesion of infected erythrocytes to CD36 correlated with high concentrations of IL-10 and low concentrations of IL-12 in plasma.     

Functional characterization of protective CD4+ T-cell clones reactive to the murine malaria parasite Plasmodium chabaudi.

Protective immunity to asexual malaria parasites appears to be mediated predominantly by the CD4+ subset of T lymphocytes. To examine the role of this T-cell population in the immune response to the murine malaria parasite Plasmodium chabaudi, CD4+ clones derived from infected mice were raised and propagated in vitro. Analysis of the reactivity of clones responsive to parasite antigen demonstrated that the CD4+ cell response is heterogeneous and is consistent with the idea of two functionally distinct CD4+ subsets. Those populations derived early during primary infection secreted interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) upon antigenic stimulation in vitro, i.e. they had a cytokine repertoire typical of the delayed-type inflammatory T-helper 1 (Th1) CD4+ subset. In contrast, cells taken after clearance of a secondary infection produced IL-4 and acted as effective helper cells for anti-malarial antibody (Ab) synthesis in vitro, and thereby had the characteristics of Th2 cells. The appearance in vivo of Th1 and then Th2 clones specific for P. chabaudi-parasitized erythrocytes (pRBC) supports the proposal from limiting culture analyses that for this malaria parasite resolution of primary parasitaemia is predominantly through the action of cytokines rather than Ab, and that final clearance requires helper cells and specific immunoglobulin.     

Recognition of Histoplasma capsulatum yeast-cell antigens by human lymphocytes and human T-cell clones.

We have prepared a detergent extract from the cell wall and cell membrane of Histoplasma capsulatum yeast cells that is recognized by T cells from mice immunized with viable organisms or with the extract. A 62-kd antigen from this extract has been isolated and has been shown to be antigenic and to confer protective immunity in mice. In this study, we examined the in vitro proliferative response by human lymphocytes and human T-cell clones to both the extract from the cell wall and cell membrane and the 62-kd antigen, termed HIS-62. Seven healthy individuals were identified whose peripheral blood mononuclear cells responded to the cell wall and cell membrane extract from H. capsulatum yeast cells. Mononuclear cells from all seven individuals recognized histoplasmin. T-cell clones were generated from peripheral blood mononuclear cells from a single person using the extract from cell wall and cell membrane as the antigenic preparation. Nineteen clones were propagated; all expressed the surface phenotype CD3+, CD4+, T-cell receptor alpha/beta +. The clones were antigen-specific. Of 17 clones studied, none responded to extracts from Blastomyces dermatitidis or Coccidioides immitis or to tetanus toxoid. Sixteen of 19 clones recognized HIS-62 in vitro. Additional analysis revealed that cells from each of the seven individuals who responded to the extract mounted a proliferative response to HIS-62. Thus, the extract from the cell wall and cell membrane and HIS-62 are targets of the human cell-mediated immune response to H. capsulatum. Moreover, HIS-62 appears to be an immunodominant antigen.     

Neonatal invariant Valpha24+ NKT lymphocytes are activated memory cells

NKT cells are a small subset of T lymphocytes which express an invariant V(alpha24JalphaQ TCR and recognize glycolipids presented by CD1d. In adults, NKT cells have a memory phenotype, frequently associated with oligoclonal expansion, express NK cell markers, and produce TO cytokines upon primary stimulation. Because of these features, NKT cells are regarded as lymphocytes of innate immunity. We investigated NKT cells from cord blood to see how these cells appear in the absence of exogenous stimuli. We found that NKT cells are present at comparable frequencies in cord blood and adult peripheral blood mononuclear cells and in both cases display a memory (CD45RO+CD62L-) phenotype. However, neonatal NKT cells differ from their adult counterparts by the following characteristics: (1) they express markers of activation, such as CD25; (2) they are polyclonal; (3) they do not produce cytokines in response to primary stimulation. Together, our data show that human NKT cells arise in the newborn with an activated memory phenotype, probably due to recognition of an endogenous ligand(s). The absence of oligoclonal expansion and primary effector functions also suggest that neonatal NKT cells, despite their activated memory phenotype, require a further priming/differentiation event to behave as fully functional cells of innate immunity.     

T-cell receptor beta gene rearrangements in clones derived from human CD4-8- cells expressing natural killer cell activity.

Clones derived from highly purified human peripheral blood Leu 19+ cells in the presence of phytohaemagglutinin (PHA) and interleukin-2 (IL-2) expressed cytotoxic activity against natural killer (NK)-resistant as well as NK-sensitive targets. All 66 clones analysed had a germ line configuration of T-cell receptor (TCR) beta genes and 38/40 also had unrearranged TCR gamma genes. The two exceptions were both CD3+ clones, but these did not have a cytotoxic repertoire noticeably different from CD3- clones without TCR gamma gene rearrangements. Clones were also obtained from highly purified CD4-8- cells, most of which were also cytotoxic for NK-resistant and NK-sensitive targets. About 90% of these clones were CD3+ but only around 50% remained negative for CD4 and CD8 while a significant number (12.7%) were positive for both CD4 and CD8. All clones analysed had rearranged TCR gamma genes and most had also rearranged TCR beta genes, including 20/25 of the clones which were CD3+4-8-. Many of the clones showed two rearrangements of TCR beta genes, and 3/4 CD3- clones had rearranged TCR beta as well as TCR gamma genes. There was no correlation between cytotoxic activity and TCR gene status or phenotype of these CD4-8- derived clones, except that clones which were Leu 19+ tended to have higher cytotoxic activity against NK-sensitive and NK-resistant targets than Leu 19-clones. The results strongly indicate that TCR beta and gamma gene products are not involved in the cytotoxicity mediated by these clones. They also suggest that some CD4-8- cells may be capable of limited differentiation in vitro.     

Clonal analysis of T cell infiltrates in synovial tissue of patients with rheumatoid arthritis

To investigate the possibility of the presence of disease-relevant, antigen-specific immune reactions in rheumatoid arthritis (RA), clonal diversity of the T cells in the synovial tissue was examined. T cells were directly cloned by in vitro stimulation with phytohemagglutin and interleukin 2 from both the peripheral blood and inflammatory synovial tissue. Their clonotypes were defined by analyzing rearrangement patterns of T cell receptor (TCR) beta and gamma chain genes using Southern blotting. In total, 111 clones from the synovial tissue (four patients) and 45 from the peripheral blood (one patient) were studied. Although most of the clones were unique in their TCR gene rearrangement patterns, 2 clones from the synovial tissue of one patient had identical patterns. These 2 clones were CD3+, 4-, 8+. Since phenotypic analysis of 82 clones from the synovial tissues revealed that CD8+ T cell clones were less frequent (24%) than CD4+ clones, the clonal identity observed here in 2 clones may not be negligible. Furthermore, 1 CD8+ clone from the peripheral blood of the same patient also had the same clonotype. These results may suggest selective trafficking or proliferation of CD4-, CD8+ T cells in RA synovial tissue.     

Molecular fingerprinting reveals non-overlapping T cell oligoclonality between an inflamed site and peripheral blood.

We have demonstrated a stable expansion of CD8+ T cells in the peripheral blood of a child with chronic arthritis. The expanded TCRBV family (TCRBV14) was enriched for CD57hiCD28- T cells. Sequencing of the TCRBV14 amplification products showed a TCR sequence which contributed 32% of the total TCR in the CD8+TCRBV14 population. Using the modified heteroduplex technique, the CD8+TCRBV14 cells showed a clonal pattern and these bands were restricted to the CD28- population. This method also detected multiple other clones within the CD8+ population but few in the CD4+ cells. The dominant TCRBV14+ clone was not detectable in synovial fluid T cells from two inflamed joints by CDR3 length analysis or heteroduplex probing, suggesting that this long-lived clone is excluded from inflammatory sites. Synovial fluid T cells showed an unexpected discordance of the CD28 and CD57 phenotype compared to peripheral blood mononuclear cells. T cells from both inflamed joints both showed marked oligoclonality in all TCR families and had almost identical heteroduplex patterns. Taken together these data suggest that some clones are actively excluded from inflamed sites in juvenile chronic arthritis, yet the pattern of restricted T cell expansion is shared between sites of inflammation.     

CD8beta/CD28 expression defines functionally distinct populations of peripheral blood T lymphocytes

Peripheral blood CD8+ T lymphocytes generally express the CD8 coreceptor as an alphabeta heterodimer. On these cells, the CD8beta chain is present either at high (CD8betahigh) or low density (CD8betalow). CD8betahigh cells are CD28+, whereas CD8betalow cells are CD28+ or CD28-. Therefore, three subpopulations of CD8+ T cells can be described: (i) CD8betahighCD28+ (ii) CD8betalowCD28+, and (iii) CD8betalowCD28- cells. Phenotypic and functional characterization of these CD8+ T cell subsets revealed significant differences. CD8betahighCD28+ cells predominantly express CD45RA. In contrast, CD8betalowCD28+ cells frequently express CD45R0 and the activating NK receptor CD161. CD8betalowCD28- cells frequently revert to the CD45RA phenotype. In addition, these cells express CD16, CD56, CD94, and the killer-inhibitory receptors NKB1 and CD158a. Intracellular IL-2 was frequently detected in CD8betahighCD28+ cells and CD8betalowCD28+ cells, but not CD8betalowCD28- cells. CD8betalowCD28+ cells and CD8betalowCD28- cells frequently stained positive for IFN-gamma. In addition, these cells contain intracellular perforin and granzyme A. Expression of Fas (CD95) as well as susceptibility to apoptosis is markedly increased in CD8betalowCD28+ and CD8betalowCD28- cells as compared to CD8betahighCD28+ cells. In vitro activation of peripheral blood lymphocytes triggered expansion of CD8betahighCD28+ cells as well as a development into CD8betalowCD28+ and CD8betalowCD28- cells. Similarly, activation of CD8betahighCD28+ cord blood cells resulted in the appearance of CD8betalowCD28+ and CD8betalowCD28- cells. These data suggest that CD8betahighCD28+ cells can differentiate into CD8betalowCD28+ and CD8betalowCD28- cells upon TCR stimulation. Therefore, the CD8beta/CD28 subsets in peripheral blood may reflect distinct stages of post-thymic CD8+T cell development.     

Splenic gammadelta T cells regulated by CD4+ T cells are required to control chronic Plasmodium chabaudi malaria in the B-cell-deficient mouse

Little is known about the function and regulation of splenic gammadelta T cells during chronic Plasmodium chabaudi malaria. The splenic gammadelta T-cell population continues to expand, reaching levels equal to 4 times the number of splenocytes in an uninfected mouse. Splenic gammadelta T cells from J(H)-/- mice with chronic malaria expressed Vgamma1+ or Vdelta4+ in the same ratio as uninfected controls with Vgamma1 cells dominating, but the Vgamma2 ratio declined about twofold. Gammadelta T cells from G8 mice specific for the TL antigen increased only 2-fold in number, compared with 10-fold in BALB/c controls, but G8 gammadelta T cells failed to express the B220 activation marker. Elimination of the parasite by drug treatment caused a slow depletion in the number of splenic gammadelta, CD4+, and CD8+ T cells. Following challenge, drug-cured J(H)-/- mice exhibited nearly identical parasitemia time courses as na?ve controls. Depletion of either CD4+ T cells or gammadelta T cells from chronically infected J(H)-/- mice by monoclonal antibody treatment resulted in an immediate and significant (P < 0.05) exacerbation of parasitemia coupled with a marked decrease in splenic gammadelta T-cell numbers. The number of CD4+ T cells, in contrast, did not decrease in mice after anti-T-cell receptor gammadelta treatment. The results indicate that cell-mediated immunity against blood-stage malarial parasites during chronic malaria (i) requires the continued presence of blood-stage parasites to remain functional, (ii) is dependent upon both gammadelta T cells and CD4+ T cells, and (iii) lacks immunological memory.