In vivo induction of CD4+ T cell responses by antigens covalently linked to synthetic microspheres does not require adjuvant

Macrophages, dendritic cells or B lymphocytes have been shownto play a major role in the presentation of soluble antigensto CD4⁺ T cells. In contrast, the capacity of these cells topresent particulate antigens such as bacterial or parasiticantigens to T cells remains controversial. To investigate thisquestion, well defined particulate antigens were prepared bycovalent linkage of proteins or peptides to 1 µm in diametersynthetic microspheres. The T cell immunogenicity of such particulateantigens was analyzed in vitro and in vivo. In vitro, a solubleprotein such as hen egg lysozyme (HEL) coupled to beads stimulateda strong proliferative T cell response of lymph node cells fromHEL-primed mice or of specific T cell hybridomas. HEL coupledto beads was presented to the specific T cell hybridomas bysplenocytes or by peritoneal macrophages, but not by lymphomaB cells. Immunization of mice with several different proteinantigens or with a synthetic peptide covalently linked to beadsinduced strong CD4⁺ T cell responses in the absence of adjuvant.The strong in vivo immunogenicity of proteins coupled to beadsdid not result from a non-specific adjuvant effect of beadssince covalent linkage of the antigen to beads was strictlyrequired to induce T cell responses in the absence of adjuvant.In vivo treatment by carrageenan showed that macrophages arerequired for the in vivo stimulation of T cell responses bythese particulate antigens. Thus, these results demonstratedthe role of phagocytic cells, especially macrophages, for invivo presentation of particulate antigens. These particulateantigens represent an interesting approach for the developmentof new vaccines, and for the in vivo analysis of the role ofvarious antigen presenting cells in T cell activation and differentiation.     

Differential presentation of hepatitis B S-preS(2) particles and peptides by macrophages and B-cell like antigen-presenting cells.

Different cell types, including dendritic cells, macrophages and Ia+ B cells, have been described to present soluble antigen (Ag) to T-cell hybridomas. However, it is still not clear whether these different cell types can act as antigen-presenting cells (APC) for complex and insoluble Ag such as viral particles. Using yeast recombinant hepatitis B S-preS(2)-containing particles, T-cell hybridomas were generated and used as a tool to study processing and presentation of antigen. Different types of APC were compared in regard to their capacity to process and present the protein-lipid composed S-preS(2) particles and the thereof derived T-cell epitope containing peptides by different types of APC. While a S-preS(2)-derived T-cell epitope containing peptide, which does not require processing, could be presented both by macrophage and B-cell like APC, the presentation of S-preS(2) particles required the presence of macrophages. The fact that B-cell like APC and macrophages behave differently with regard to the presentation of S-preS(2) particles suggest that the uptake and/or processing of this type of Ag by B-cell like APC and macrophages is different.     

Studies on the relationships between the immunogenicity and catabolism of antigens and their binding to the surface of macrophages

The relationship between immunogenicity of Shigella paradysenteriae, the branched synthetic polypeptide poly-L (Tyr, Glu)-polyL Pro-polyL Lys [(T, G)-Pro–L] and human serum albumin (HSA) interacting with macrophages and the kinetics of antigen degradation and degree of binding to the cell surface was studied. Following thioglycollate inoculation into C57BL/6 mice, the peritoneal-stimulated macrophages had higher levels of hydrolases as compared to unstimulated cells. The lysates of the stimulated macrophages catabolized the three labeled antigens faster than did the lysates of unstimulated cells. However, when degradation of labeled antigens by macrophage cells was assessed, no direct correlation could be demonstrated between the level of cell hydrolases and rate of (T, G)-Pro–L or HSA catabolism. The immunogenicity of the antigens following their uptake by unstimulated and stimulated macrophages was determined by transfer of the antigen-bearing cells into irradiated and nonirradiated syngeneic recipients. No correlation was apparent between the rate of antigen degradation and the capacity to evoke a humoral response. Similarly, no correlation could be demonstrated between the amount of antigen bound to the macrophage cell surface and the immunogenicity of the antigen. It is suggested that neither the rate of antigen catabolism by macrophages nor the amount of antigen bound to the macrophage membrane is the sole factor which determines the immunogenicity of antigens interacting with macrophages.     

Processing of normal antibody-coated sheep erythrocytes by mouse peritoneal macrophages

The processing by mouse peritoneal macrophages of normal sheep red blood cells (SRBC) or SRBC coated with 19S or 7S antibody was investigated by measuring (a) total uptake of SRBC, (b) changes in subcellular distribution of antigen during in vitro culture, and (c) immunogenicity of SRBC-containing macrophages. Most of the ingested antigen was found to be bound to lysosomal membranes. Its concentration decreased rapidly during the first 5 hours of culture. Although SRBC uptake was increased by the addition of either class of antibody, 7S antibody suppressed and 19S antibody enhanced the immunogenicity of SRBC-containing macrophages. Rate of intracellular antigen degradation and production of low-density antigen did not correlate with macrophage immunogenicity. While the enhancing effect of 19S antibody probably was due to an increase in the percentage of SRBC-ingesting cells, suppression by 7S antibody may have been related to the creation of a distinct class of heavily laden macrophages with reduced immunogenic efficiency.     

Endogenous and exogenous forms of the same antigen are processed from different pools to bind MHC class II molecules in endocytic compartments

The current studies were carried out to examine the basis for the differences in the antigenic peptides generated from exogenous and endogenous forms of hen egg white lysozyme (HEL). The role of different intracellular compartments in the generation and binding of HEL peptides derived from two endogenous forms of HEL, either secreted (sHEL) or retained in the endoplasmic reticulum (ER, KDEL HEL), presented by MHC class II molecules was examined and compared to exogenous HEL. Initially it was found that antigen-presenting cells bearing both intracellular forms of HEL generated and presented a number of IA^k-restricted HEL epitopes to T cell hybridomas, although s HEL was processed more efficiently than KDEL HEL. There were differences, however, for some determinants between endogeneous and exogenous HEL. At equivalent antigen-presenting efficiencies, endogenous HEL-bearing cells displayed a lower surface density of IA^k-bound HEL-52-61-related peptides than cells pulsed with exogenous HEL, as detected by a specific monoclonal antibody. Neither endogenous HEL degradation nor peptide binding to MHC class II molecules occurred in the ER. Processing of sHEL and KDEL HEL appears to take place either in a post-trans-Golgi network acidic compartment or in the cytosol, whereas peptide binding to MHC class II molecules occurs in endocytic compartments . Furthermore, the peptides generated were derived from an endogenous source rather than from secreted and re-endocytosed HEL. Thus, processing of endogenous HEL is from a different pool than exogenous HEL and occurs in different compartments.     

High efficiency of endogenous antigen presentation by MHC class II molecules.

MHC class II molecules are involved in the presentation of both exogenous and endogenous antigens to CD4 T cells. Using the trans-membrane hemagglutinin (HA) from measles virus and the secreted hen egg lysozyme (HEL) as antigen models, we have compared the efficiency of MHC class II presentation by naive antigen presenting cells (APCs) pulsed with exogenous antigen with that of their transfected counterparts synthesizing endogenous antigen. B cells expressing even a very low amount of trans-membrane HA were found to present endogenous HA to I-Ed restricted T cell hybridomas with a high efficiency whereas their naive counterparts required to be pulsed with a comparatively high amount of exogenous HA. Similarly, MHC class II presentation of endogenous secreted HEL was found to be much more efficient when compared with that of exogenous HEL. Biochemical studies did not reveal any enhanced intracellular degradation of endogenous HEL. As expected, HEL was released in the surrounding medium within < 1 h. MHC class II presentation of endogenous HEL could not be explained by re-uptake by bystander APCs of HEL secreted in the surrounding medium. No sensitization of naive APCs could be observed either when co-cultured with HEL secreting cells or when cultured for 10 days with a sub-threshold amount of exogenous HEL. At the cell surface, I-Ed molecules immunoprecipitated from HEL secreting cells were found to be slightly enriched in SDS-resistant forms. These data raised the question of how peptides derived from endogenous transmembrane and secreted antigens can so efficiently reach an MHC class II loading compartment.     

Processing and presentation of intact hen egg-white lysozyme by dendritic cells.

Dendritic cells in lymphoid tissues are of key importance as highly specialized antigen-presenting cells for the induction of T lymphocyte responses. Conflicting results have been published regarding antigen processing of intact proteins by dendritic cells. We now report that highly purified dendritic cells isolated from H-2k mouse spleens very efficiently generated immunogenic fragments of intact hen egg-white lysozyme (HEL) protein to present to an I-Ak-restricted T hybridoma cell line, specific for HEL peptide 46-61. Dendritic cells required 100 times less HEL protein than lipopolysaccharide-induced B cell blasts for effective presentation. Uptake of 125I-labeled HEL protein by dendritic cells and inhibition of presentation of HEL protein by chloroquine treatment was observed. This indicates an endocytotic process and the involvement of acidified compartments. Since the supernatant of dendritic cells, that were incubated with intact HEL protein, contained immunogenic fragments, further evidence for processing of HEL protein by dendritic cells was obtained. When HEL protein was covalently coupled to beads, dendritic cells were not able to ingest these beads, but could still process HEL protein for presentation. This suggests cell surface processing of HEL protein, although internalization of HEL protein released from the beads cannot be excluded. Taken together, these data show that H-2k dendritic cells are capable of processing and presenting intact HEL protein.     

Cathepsin D,but not cathepsin B,releases T cell stimulatory fragments from lysozyme that are functional in the context of multiple murine class II MHC molecules

In this study, the major endosomal/lysosomal proteases cathepsin D and cathepsin B were tested on their ability to release T cell stimulatory peptides from hen egg white lysozyme (HEL) in vitro. Whereas neither enzyme could cleave unreduced HEL under mild conditions, reduced HEL was readily cleaved by cathepsin D but not by cathepsin B. Instead, cathepsin B was found to be very active in the trimming of HEL peptides after their release by cathepsin D. Following high-performance liquid chromatography (HPLC) fractionation, cathepsin D-released HEL fragments were screened for recognition by HEL-specific T cells from three strains of mice, i.e. B10. A (H-2^a), C57BL/6 (H-2^b) and BALB/c (H-2^d). Peptides in a large number of different HPLC fractions triggered significant T cell responses in all three strains. Interestingly, the response profiles of T cells from the three different strains showed marked similarities. Also, several individual synthetic HEL sequences corresponding to selected cathepsin D-released fragments were recognized by murine T cells in the context of all three major histocompatibility complex (MHC) haplotypes tested. Our data suggest that cathepsin D rather than cathepsin B may play a central role in the initial release of HEL fragments during endosomal/lysosomal processing. The relatively long HEL fragments released by cathepsin D, containing about 20—30 amino acid residues, are significantly more promiscuous in murine class II MHC binding than the shorter synthetic HEL sequences previously employed by others for the delineation of HEL epitopes. Extensive documentation of HEL epitopes in previous investigations indicate that this promiscuity cannot be explained by simply assuming that longer peptides contain additional epitopes. Rather, an increased peptide length by itself appears to promote promiscuous MHC binding.     

Processing and presentation of hen egg-white lysozyme by macrophages

The processing and presentation by macrophages of the well-defined protein hen egg-white lysozyme (HEL) was analyzed using two HEL-specific T cell hybridomas. The processing studies revealed that both clones required that native HEL be processed, while neither clone required any processing of a tryptic digest of lysozyme. A differential requirement for processing was found for the intact, denatured lysozyme (CM-HEL) with one clone (2A11) requiring processing, and a second clone (3A9) did not require any processing. The determinant on the HEL molecule that both clones recognized was localized to a tryptic fragment containing residues 46 to 61. By testing the immunogenicity of fragments of the 46-61 peptide, mouse lysozyme, and human lysozyme, we were able to localize the T cell determinant to either of two residues, Gly-49 or Leu-56.     

The role of macrophages in the generation of T helper cells. III. Influence of macrophage-derived factors in helper cell induction

Helper cell induction to soluble or particulate antigens in vitro requires the cooperation of T cells and macrophages. A direct contact between macrophages and T cells is not obligatory for this cooperation and factors released from macrophages are as effective in activating T cells as the cells themselves. Two different types of macrophage-derived factors where found. The supernatant obtained from purified macrophages incubated with antigen for several days generates helper cells in absence of macrophages or additional antigen, but only if obtained from macrophages which were identical at the I-A subregion of the H-2 complex as the T cells. This factor was called genetically related macrophage factor (GRF). The other factor(s), which is present in the supernatant obtained from macrophages incubated for several days without antigen, replaces macrophages only if the antigen is particulate. This factor(s), called nonspecific macrophage factor (NMF) is not restricted genetically and is also obtained from allogeneic macrophages. The importance of both these factors in helper cell induction is discussed.     

Minute quantities of a single immunodominant foreign epitope are presented as large nested sets by major histocompatibility complex class II molecules

The processing and presentation of immunogenetic peptides is an obligate event in the generation of an immune response. However, the degree of complexity with which an immunogenic foreign epitope is presented is still unclear. This question was addressed by analyzing the naturally processed peptides generated from exogenously-derived hen egg white lysozyme (HEL) bound to the murine major histocompatibility complex (MHC) class II molecule, H-2A^k. Using reversed-phase chromatography (RPC), T cell hybridomas and mass spectrometry, 16 peptides were identified that contain the minimal MHC binding epitope 52–61. These peptides exhibited substantial N- and C-terminal extensions and ranged from 13–28 amino acids in length. In contrast, MHC class I molecules present peptides of 8–11 residues and each foreign epitope appears to be represented by only a single peptide. The data here also show that only ～ 0.8% of the total bound peptide was derived from this single HEL epitope. These findings provide direct evidence that relatively small amounts of processed peptide are required to stimulate an effective T cell response.     

Promiscuous T cell recognition of an H-2 IA-presented mycobacterial epitope

Genetically permissive T cell epitopes are an important prerequisite for the development of peptide-based vaccines or immunodiagnostic reagents. We have investigated the structural requirements of permissive T cell recognition of peptide p350—369 from the 38-kDa antigen of Mycobacterium tuberculosis. This peptide was found to be immunogenic in mice of the H-2^{b, bm12, d. s and k}, but not of the H-2^f genotype. T cell responses were restricted by I-A class II molecules. The same epitope core was recognized in the H-2^{b, d and k} genotypes. T cell hybrids from BALB/c and C57BL/10 mice were used to determine: (i) the critical residues using substituted peptide derivatives and (ii) the degree of T cell promiscuity. Two out of five BALB (H-2^d)-derived hybridomas tested displayed promiscuous peptide recognition in the context of H-2^b and H-2^bm12 antigen-presenting cells. The recognition of critical residues was found to be uniform for all five hybridomas when tested with syngeneic antigen-presenting cells; additional critical residues were identified when the peptide was recognized in the context of allogeneic antigen-presenting cells. Only one of the four tested C57BL/10 (H-2^b) hybridomas showed promiscuity in the context of H-2^bm12. Each of these C57BL/10-derived clones had a distinct response profile toward the critical residues. We propose that the demonstrated T cell promiscuity involves peptide interaction with polymorphic H-2 I-A residues.     

Analysis of T cell reactivities to phosphorylcholine-conjugated hen egg lysozyme in C57BL/6 mice: hapten-conjugate specificity reflects an altered expression of a major carrier epitope.

We have selected several different T cell hybridoma clones reactive to hen egg lysozyme (HEL) conjugated to phosphorylcholine (PC) after fusion of PC-HEL-primed C57BL/6 lymphocytes with BW5147 parent cells. These hybridoma clones preferentially recognize PC-HEL over unconjugated HEL, but not other carrier molecules conjugated with the same hapten. All the PC-HEL-reactive clones are similarly responsive to not only p-azobenzenearsonate (ABA)-conjugated HEL (ABA-HEL) but also to a variety of other diazotized hapten-HEL conjugates. However, these clones are not stimulated by fluoresceinated or dinitrophenylated HEL beyond the level of HEL carrier alone. Therefore, the type of hapten linkage (diazonium) to the carrier molecule appears to affect T cell recognition. The hybridoma clones apparently recognize the carrier molecule alone, although the level of stimulation is relatively low compared to that induced by either PC-HEL or ABA-HEL. Interestingly, HEL unfolded by S-carboxymethylation is capable of stimulating the hybridomas to a level comparable to that obtained with PC-HEL. T cell recognition of the unfolded HEL is independent of antigen processing, which is different from that of PC-HEL. The peptide sequence corresponding to the amino acids 81-93 of HEL appears to contain the epitope region for the hybridoma clones based on testing stimulation activity with synthetic peptides. Previously, the peptides including this region (81-96) have been reported as the determinant recognized by T cells derived from C57BL/6 mice after immunization with an HEL peptide (HEL 13-105) but not with native HEL. These results suggest that a hapten conjugation via diazonium linkage modifies antigen presentation and consequently the presentation of the major T cell epitopes similar to that of the HEL fragment.     

TAP1-independent loading of class I molecules by exogenous viral proteins

Presentation of peptides derived from endogenous proteins on class I molecules needs functional TAP peptide transporters. To reveal whether class I-associated presentation of exogenous proteins also required the presence of TAP transporters, we assessed in vitro the ability of spleen cells and macrophages from TAP1-deficient mice (TAP1?/?) to present peptides derived from exogenous recombinant viral proteins on their class I molecules. We found that recombinant glycoand nucleoprotein from lymphocytic choriomeningitis virus and nucleoprotein of vesicular stomatitis virus were presented as efficiently by TAP1 ?/? cells as by control cells. Peptide regurgitation was not involved. Since particulate, nonreplicating antigens can efficiently prime anti-viral cytotoxic T cells in vivo, this new, TAP-independent pathway of class I-associated antigen presentation may be applicable for vaccine strategies.     

Antibody synthesis by chicken spleen cells in vitro. I. Requirements of B cells at various stages after immunization for T cells,macrophages and antigen

The requirements for T cells, macrophages and antigen during the induction of in vitro antibody responses were ascertained with chicken spleen cells obtained at various times after immunization with sheep red blood cells (SRBC). The IgM plaque-forming cell (PFC) response was T cell independent exclusively in cultures initiated 3 days after priming, but macrophage dependent at all time intervals tested. In cultures started 4 to 10 days after priming the IgG response was both T cell and macrophage independent and PFC numbers remained at a high plateau level throughout the culture period. In contrast, IgG responses initiated more than 15 days after priming showed a reversal to complete T cell and macrophage dependence and were characterized by a sharp increase in PFC numbers between days 2 and 4 of culture. Formaldehyde-fixed SRBC were immunogenic for IgG PFC 4 to 10 days after priming but failed to stimulate later IgG memory and all IgM responses. Contrasting antigen dose requirements for IgM and IgG responses were found in cultures initiated at various periods after priming. The results suggest that direct contact with fixed antigen was sufficient to maintain IgG antibody synthesizing PFC in vitro, while native antigen and cell co-operation were required for late secondary IgG and all IgM responses. These results are interpreted in terms of separate pathways of differentiation for IgM and IgG antibody-producing cells. A distinct 3rd day stage of T cell-independent but macrophage-dependent responsiveness for both classes of antibody was also defined.     

Cytosolic targeting of hen egg lysozyme gives rise to a short-lived protein presented by class I but not class II major histocompatibility complex molecules

A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen-presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence-deleted HEL cDNA, respectively. HEL was evidenced in both HELs- and HELc-transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half-life of less than 5 min. HEL-derived peptides could not be shown biochemically either in HELc- nor in HELs-transfected cells. We then studied the capacity of these cells to present HEL to HEL-specific class I- and class II-restricted T cells. Both cell types could be recognized by the HEL-specific MHC class I-restricted CTL clones. In contrast, MHC class II-HEL peptide complexes, recognized by HEL-specific helper T cell hybridomas, could be detected on MHC class II+ HELs- but not HELc-transfected cells. In vivo experiments showed, however, that HELc-transfected cells could provide host APC with HELc-derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II-HELc-transfected cells, unable by themselves to elicit any anti-HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.     

Anergic B cells constitutively present self antigen: Enhanced immunoglobulin receptor-mediated presentation of antigenic determinants by B cells is hierarchical

Presentation of hen egg lysozyme (HEL) by HEL-specific B cells was studied in transgenic mice expressing anti-HEL immunoglobulin (Ig-transgenic). In T hybridoma assays, presentation of the HEL_46–61 determinant by B cells from Ig-transgenic mice required 10³?10⁴-fold lower concentrations of HEL than were required for presentation by B cells from non-transgenic mice. In contrast, presentation of the HEL determinants 112–129 and 25–43 by HEL-specific B cells was either not significantly enhanced, or enhanced only 10-fold compared with B cells from non-transgenic mice. Enhanced presentation of HEL determinants by B cells from Ig-transgenic donors was specific for HEL, since keyhole limpet hemocyanin or synthetic HEL_46–61 peptide were presented comparably by B cells from Ig-transgenic mice and non-transgenic littermates. A minimum of 1–4% Ig-transgenic B cells was required to detect enhanced presentation of HEL_46–61 in vitro. Constitutive presentation of the HEL_46–61 determinant, but not the HEL_25–43 or HEL_112–129 determinants, was detectable on anergic HEL-specific B cells from double (HEL/Ig)-transgenic mice. In the presence of exogenously added HEL, anergic B cells presented all three HEL determinants. Constitutively presented HEL_46–61 was not due to endogenous synthesis of HEL antigen by anergic B cells from double-transgenic mice, as comparable levels of the HEL_46–61 determinant were constitutively presented by B cells from Ig-Tg → HEL-Tg irradiation bone marrow chimeric mice. Firstly, these results indicate that the enhanced antigen presentation mediated by Ig receptors on B cells is not equivalent for all antigenic determinants. Secondly, the data demonstrate that anergic, autoreactive B cells efficiently process and present nominal antigens in addition to constitutively presenting specific self antigen in vivo.     

Similar as well as distinct MHC class I-binding peptides are generated by exogenous and endogenous processing of hepatitis B virus surface antigen

Murine MHC class I-restricted cytotoxic T lymphocyte (CTL) responses can be primed by exogenous as well as endogenous hepatitis B surface antigen (HBsAg). Immunodominant CTL-defined epitopes of this viral envelope protein are the L^d -binding 12-mer S28 – 39 peptide IPQSLDSWWTSL in H-2 ^d mice, and the K^b -binding 8-mer S208 – 215 peptide ILSPFLPL in H-2^b mice. We tested if CTL recognizing these epitopes can be primed in vivo by HBsAg delivered as either an exogenous antigen (native HBsAg lipoprotein particles), or an endogenous antigen (plasmid DNA encoding HBsAg). Primed T cells were restimulated in vitro prior to the cytotoxicity assay with cells presenting the H-2 class I-binding epitopes generated by either exogenous or endogenous processing of HBsAg. The data indicate that the L^d -binding peptide S28 – 39 is generated during exogenous as well as endogenous processing of HBsAg. In contrast, the K^b -binding peptide S208 – 215 is generated during exogenous but not endogenous processing of HBsAg. Hence, some but not all MHC class I-binding, immunogenic peptides are generated during endogenous and exogenous processing of HBsAg but there also exists a repertoire of immunogenic peptides of viral origin that is only revealed after exogenous processing of viral proteins.     

Dendritic cell/macrophage precursors capture exogenous antigen for MHC class I presentation by dendritic cells

Presentation of MHC class I antigens by professional antigen-presenting cells (APC) is an important pathway in priming cytotoxic T lymphocyte responses in vivo. This study sought to identify the nature of the professional APC responsible for indirect class I presentation by examining a special feature of professional APC, namely their ability to process exogenous forms of antigen for class I presentation. Incubation of highly purified bone marrow-derived precursor cells with chicken ovalbumin (OVA) led to the efficient presentation of the major class I-restricted OVA determinant by mature dendritic cells (DC), but not by macrophages (Mϕ) derived from the precursor population. DC as well as macrophages were, however, able to mediate class II presentation of OVA, suggesting that macrophages were deficient in class I processing but not in capturing exogenous OVA. The majority of mature DC, i.e. over 80 %, generated from the precursor cells pulsed with OVA, presented the class I OVA epitope. Upon maturation, class I presentation of OVA by DC was greatly reduced, suggesting that class I processing of exogenous antigen is modulated during DC maturation in a manner similar to class II antigen processing. This study shows that bone marrow-derived DC/Mϕ progenitors capture exogenous antigen for class I presentation, and that cells of the DC lineage can be functionally distinguished from cells of the macrophage lineage based on their ability to process exogenous antigen for class I presentation.     

GENETIC CONTROL OF CELL-MEDIATED IMMUNITY IN THE RAT:II. SHARING OF EITHER THE RTI. A OR RTI. B LOCUS IS SUFFICIENT FOR TRANSFER OF ANTIMICROBIAL RESISTANCE

The MHC restriction criteria for T cells activating macrophages in vivo and mediating antimicrobiol resistance to Listeria monocytogenes were determined. Antimicribiol resistance could be transferred by T cell in order of decreasing efficiency from syngeneic. RT1A. compatible, RTI.B compatible and RT1 incompatible donors. Alloreactive T cells responding to either A locus or B locus encoded antigens in a graft-versus-host reaction were also able to activate macrophages. Approximately five times as many MLC-reactive precursors responded to B locus alloantigens as to A locus alloantigens, but A-resticted Listeria-specific T cells wre considerably more numerous (or more efficient) in Listeriainfected hosts than were B-restricted, Listeria-specific T cells. This was unexpected, since A-restricted, Listeria-specific T cells failed to transfer delayed hypersensitivity (DTH) to soluble bacterial antigens.