A point mutation in the human transporter associated with antigen processing (TAP2) alters the peptide transport specificity

The heterodimeric transporter associated with antigen processing (TAP1/TAP2) translocates peptides from the cytosol into the endoplasmic reticulum where loading of major histocompatibility complex class I molecules takes place. TAP transporters from different species are known to exhibit distinct transport specificities with regard to the C-terminal amino acid (aa) of peptides. Thus, human TAP (hTAP), and rat TAP (rTAP) containing the rTAP2^a allele are rather promiscuous, whereas mouse TAP (mTAP), and rTAP containing the rTAP2^u allele are restrictive and select against peptides with C-terminal small polar/hydrophobic or positively charged aa. The structural basis for this selectivity is not clear. To assess the relative contribution of the TAP1 and TAP2 subunits to transport specificity, we have constructed and analyzed interspecies TAP hybrids and point mutants of hTAP2 expressed in Sf9 insect cells and in TAP-deficient T2 cells. Transport assays with 20 C-terminal variants of the peptide RYWA-NATRSX showed that: first, transport specificity with regard to C-terminal aa is mainly influenced by TAP2, but TAP1 can also contribute. Second, the selective transport of peptides with C-terminal positively charged aa is critically controlled by the amino-terminal region (1–361) on the TAP2 chain, while transport of peptides with C-terminal small polar/hydrophobic aa is determined by residues located within as well as outside the region 1–361. Third, a single point mutation in hTAP2 (374A → D) resulted in a drastic alteration of the transport pattern. These results indicate that both TAP1 and TAP2 contribute to efficient peptide transport and that single point mutations in hTAP2 are able to alter the peptide transport specificity. This opens the possibility that naturally occurring mutations in one of the hTAP subunits may alter epitope selection in vivo.     

Translocation of long peptides by transporters associated with antigen processing (TAP)

The major histocompatibility complex (MHC)-encoded transporters associated with antigen processing (TAP) translocate peptides from the cytosol into the lumen of the endoplasmic reticulum (ER) where they associate with MHC class I molecules. The length of class I-binding peptides is usually 8–11 amino acids, but examples of significantly longer peptides have been described. The preferred lengths and upper and lower size limits for peptides translocated by TAP have not been determined in detail because in the currently used test systems, peptides are subject to proteolytic degradation. In the present study, three sets of individual peptides or partially randomized peptide libraries ranging between 6 and 40 residues were used that contained a radiolabeled tyrosine and a consensus sequence for ER-specific N-glycosylation at opposite ends, thus ensuring that only nondegraded peptides were monitored in the transport/glycosylation assay. For three different transporters, rat TAP1/2^a, rat TAP1/2^u and hTAP, the most efficient ATP-dependent transport was observed for peptides with 8–12 amino acids. Hexamers and longer peptides of up to 40 amino acids were also translocated, albeit less efficiently. For two of the three sets of peptides analyzed, rat TAP1/2^a showed a less stringent length selection than rat TAP1/2^u and human TAP. The superior transport of the decamer of the TNKT . Y series was not due to faster degradation or less efficient glycosylation of shorter or longer length variants. A binding assay with TAP-containing microsomes revealed a high affinity for the radiolabeled decamer (K_D = 580 nM), while other length variants were clearly inferior in their binding affinities. Thus, TAP binds and preferentially translocates peptides with a length suitable for binding to MHC class I molecules, but peptides that are considerably longer may also be substrates. About 10⁵ peptide binding sites per cell equivalent of microsomes were determined, providing an estimate for the number of TAP complexes in the ER membrane.     

Inhibition of natural killer cell-mediated bone marrow graft rejection by allogeneic major histocompatibility complex class I,but not class II molecules

The role of major histocompatibility complex (MHC) class I and class II molecules in natural killer (NK) cell-mediated rejection of allogeneic, semi-syngeneic and MHC-matched bone marrow grafts was investigated. The use of β₂-microglobulin (β₂m) -/- and β₂m +/- mice as bone marrow donors to MHC-mismatched recipients allowed an analysis of whether the presence of semi-syngeneic and allogeneic MHC class I gene products would be triggering, protective or neutral, in relation to NK cell-mediated rejection. Loss of β₂m did not allow H-2^b bone marrow cells to escape from NK cell-mediated rejection in allogeneic (BALB/c) or semi-allogeneic (H-2D^d transgenic C57BL/6) mice. On the contrary, it led to stronger rejection, as reflected by the inability of a larger bone marrow cell inoculum to overcome rejection by the H-2-mismatched recipients. In H-2-matched recipients, loss of β₂m in the graft led to a switch from engraftment to rejection. At the recipient level, loss of β₂m led to loss of the capability to reject H-2-matched β₂m-deficient as well as allogeneic grafts. When MHC class II-deficient mice were used as donors, the response was the same as that against donors of normal MHC phenotype: allogeneic and semi-syngeneic grafts were rejected by NK cells, while syngeneic grafts were accepted. These data suggest a model in which allogeneic class I molecules on the target cell offer partial protection, while certain syngeneic class I molecules give full protection from NK cell-mediated rejection of bone marrow cells. There was no evidence for a role of MHC class II molecules in this system.     

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.     

MUC1 peptide epitopes associated with five different H-2 class I molecules

We have previously described the induction of murine CD8⁺ major histocompatibility complex (MHC) class I-restricted cytotoxic T cells (CTL) recognizing the 20-amino acid repeat region of the human mucin 1 (MUC1) variable number of tandem repeats region (VNTR), a mucin greatly increased in expression in breast cancer and proposed as a target for immunotherapy. In that study, CTL could detect MUC1 peptides associated with the MHC of all nine strains examined, and we now report the different epitopes presented by five different MHC class I molecules. The epitopes were defined in CTL assays using peptide-pulsed phytohemagglutinin blasts or MHC class I-transfected L cells as targets; in addition, peptide binding assays and T cell proliferation studies were performed. Within the 20-amino acid VNTR, nine potential epitopes could be defined. The epitopes for the four MHC class I molecules [K^b (three epitopes), D^d, L^d and K^k] were closely related, all containing the amino acids PDTRPAP. For D^b, three epitopes were identified, all containing APGSTAP. Most of the epitopes did not contain a consensus motif for the particular MHC class I allele, and bound with low ‘affinity’, compared with known high-affinity peptides. CD8⁺ T cell proliferation also occurred to the same MHC class I-presented epitopes. Finally, when conventional anchor residues were introduced into the peptides, peptide binding increased, whereas CTL recognition was either retained (K^b) or lost (D^b) depending on the epitope.     

Major histocompatibility complex class I-restricted antigen processing and presentation

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to CD8-expressing cytotoxic T lymphocytes (CTLs). This antigen recognition system is critically important for immune surveillance against viruses and tumors. Most class I-binding peptides are generated in the cytosol, as side products from the degradation of misfolded proteins by proteasomes. A subset of the resulting peptides are translocated across the endoplasmic reticulum (ER) membrane by a dedicated peptide transporter, and these peptides are then loaded onto peptide-receptive class I molecules in the ER. The stable assembly of class I molecules with peptides is controlled by a variety of accessory proteins, including chaperones with general housekeeping functions and factors with dedicated roles in class I assembly. Peptide-filled class I molecules are then delivered to the cell surface for recognition by CTLs. This highly regulated process permits the host to rapidly counter invading pathogens with strong and sustained CTL responses and, at the same time, avoid misguided attacks. Here, how the class I antigen processing machinery accomplishes this daunting task is reviewed.     

Transporter associated with antigen processing (TAP) 1 gene polymorphisms in patients with hypersensitivity pneumonitis

Hypersensitivity pneumonitis (HP) is a lung inflammatory disease caused by the inhalation of a variety of antigens. Previous studies support the role of the major histocompatibility complex (MHC) class II genes in the susceptibility to develop HP. However, the putative role of other MHC loci has not been elucidated. Transporters associated with antigen processing (TAP) genes are located within the MHC class II region and play an important role transporting peptides across the endoplasmic reticulum membrane for MHC class I molecules assembly. The distribution of single nucleotide polymorphisms (SNPs) in TAP1 genes was analyzed in 73 hypersensitivity pneumonitis (HP) patients and 58 normal subjects. We found a significant association of the allele Gly-637 (GGC) (p=0.00004, OR=27.30, CI=3.87-548.04) and the genotypes Asp-637/Gly-637 (p=0.01, OR=16.0, CI=2.19-631.21), Pro-661/Pro-661 (p=0.006, OR=11.30, CI=2.28-75.77) with HP. A significant decrease in the frequency of the allele Pro-661 (CCA) (p=0.008, OR=0.06, CI=0-0.45), the genotype Asp-637/Asp-637 (p=0.01, OR=0.17, 95% CI=0.05-0.58) and the haplotype [Val-333 (GTC), Val-458 (GTG), Gly-637 (GGC), Pro-661 (CCA)] was detected in HP patients compared with controls (p=0.002, OR=0.07, CI=0.0-0.57). These findings suggest that TAP1 gene polymorphisms are related to HP risk, and highlight the importance of the MHC in the development of this disease.     

Constitutive macropinocytosis allows TAP-dependent major histocompatibility compex class I presentation of exogenous soluble antigen by bone marrow-derived dendritic cells

Dendritic cells expanded from mouse bone marrow (BMDC) with granulocyte/macrophage-colony-stimulating factor have potent T cell-stimulatory properties both in vitro and in vivo. This has been well documented for major histocompatibility complex (MHC) class II-restricted responses, and more recently using peptide-loaded and protein-pulsed DC for CD8 responses following adoptive transfer in mice. An unresolved question concerns the capacity of BMDC to present exogenous antigen on MHC class I molecules, an unconventional mode of MHC class I loading for which there is now considerable evidence, particularly in macrophages. Here, we show that BMDC exhibit high levels of macropinocytosis driven by constitutive membrane ruffling activity. Up to one-third of actively ruffling and macropinocytosing BMDC transferred pinocytosed horseradish peroxidase into the cytosol following a 15-min pulse, suggesting that they might be capable of presenting exogenous soluble antigen on MHC class I molecules. We show that BMDC presented exogenous ovalbumin to a T cell hybridoma more effectively, more rapidly, and at lower exogenous antigen concentrations than BM macrophages on a cell-for-cell basis. Presentation was TAP dependent, brefeldin A sensitive, and blocked by inhibitors of proteasomal processing, demonstrating use of the classical MHC class I pathway. Although effective presentation of exogenous antigen by BMDC occurred in the absence of agents which stimulate macropinocytosis, treatment with phorbol myristate acetate (PMA) enhanced both pinocytosis and MHC class I presentation by BMDC. Finally, PMA-stimulated BMDC exposed to exogenous ovalbumin in vitro were able to prime an antigen-specific cytotoxic T lymphocyte response following adoptive transfer in vivo.     

Major histocompatibility complex class I molecules are required for the development of insulitis in non-obese diabetic mice

An early step in the development of autoimmune diabetes is lymphocyte infiltration into the islets of Langerhans of the pancreas, or insulitis. The infiltrate contains both CD4⁺ and CD8⁺ T cells and both are required for progression to diabetes in non-obese diabetic (NOD) mice. It has been thought that the CD4⁺ lymphocytes are the initiators of the disease, the islet invaders, while CD8⁺ cells are the effectors, the islet destroyers. We question this interpretation because NOD mice lacking MHC class I molecules, hence CD8⁺ T cells, do not display even insulitis when expected.     

Expression of TAP1 by human trophoblast

Successful placentation in the human is dependent on the trophoblast evading recognition and destruction by the maternal immune system. However, invasive cytotrophoblast express HLA-G which may be able to present peptide to T cells. Transporter proteins are essential for peptide presentation and major histocompatibility complex (MHC) class I assembly. We have determined their expression by trophoblast in relation to HLA-G, using immunohistochemistry. Antitransporter protein antibody (TAP1) labeling closely paralleled that of MHC class I, but the intensity of its expression was much greater on the HLA-G⁺ extravillous cytotrophoblast than any other fetal or maternal tissue in the first trimester and at term. This suggests that the extravillous cytotrophoblast are very actively assembling MHC class I antigens with peptides. However, expression of MHC class I by the cytotrophoblast was not correspondingly elevated. This pattern could result from HLA-G being shed from the surface of the trophoblast, a process which may play a central role in protecting the fetus from maternal immune attack.     

Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (β₂-microglobuhn; β₂ m).The heavy chain, which comprise the actual peptide binding α-1 and α-2 domains, can exist at the cell surface in different forms, either free, bound to β₂m or as a ternary complex with β₂m and peptides. MHC-I chains are also known to internalize, and recycle to the cell surface, and this has been suggested to be important in peptide presentation. Whether MHC-I-bound peptides also can recycle is not known. We have investigated this by using both peptide transporter mutant RMA-S cells and EL4 cells loaded with D^b-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Galα4Gal) at a position which did not interfere with D^b binding or immunogenicity, and peptide recycling tested with Gal₂-specific monoclonal antibodies. By flow cytometry, a return of Gal₂ epitopes to the cell surface was found, after cellular internalization and cell surface clearance by pronase treatment. This peptide recycling could be discriminated from free fluid-phase uptake and was inhibited by methylamine, chloroquine and low temperature (18°C) but not by leupeptin. Second, specific CTL were reacted with peptide-loaded target cells after complete removal of surface D^b molecules by pronase, and after different times of incubation at 37C to allow reexpression. By this procedure, reappearance of target cell susceptibility was confirmed. The results are in agreement with a model for optimizing peptide presentation by recycling through an intracellular compartment similar to early endosomes in certain antigen-presenting cells.     

A quantitative assay to measure the interaction between immunogenic peptides and purified class I major histocompatibility complex molecules

A direct and sensitive biochemical assay to measure the interaction in solution between peptides and affinity-purified major histocompatibility complex (MHC) class I molecules has been generated. Specific binding reflecting the known class I restriction of cytotoxic T cell responses was obtained. Adding an excess of β₂-microglobulin (β₂m) significantly increased the rate of peptide association, but it did not affect the rate of dissociation. Binding was complicated by a rapid and apparently irreversible loss of functional MHC class I at 37°C which might limit the life span of empty MHC class I thereby preventing the inadvertent exchange of peptides at the target cell surface. All class I molecules tested bound peptides of the canonical octa- to nona-meric length. However, one class I molecule, K^k, also bound peptides, which were much longer suggesting that the preference of class I molecules for short epitopes is not absolute and may be caused by factors other than the peptide-MHC class I binding event itself.     

The effect of anchor residue modifications on the stability of major histocompatibility complex class I-peptide interactions

Anchor residues in peptides determine the specificity of binding to major histocompatibility complex class I molecules through interactions of their side chains with pockets in the peptide-binding groove. We have compared the kinetics of association of a Sendai virus nucleoprotein-derived peptide (FAPGNYPAL, termed SV9) with H-2K^b class I molecules, and the same peptide iodinated on the anchor residue tyrosine (¹²⁵I-SV9). Even though the association rates were too rapid for direct measurements, competition studies indicated that they were similar for SV9 and ¹²⁵I-SV9. To measure the binding of non-radioactive SV9 directly, SV9 was tritiated (³H-SV9). ³H-SV9 remained stably associated with H-2K^b molecules, whereas ¹²⁵I-SV9 dissociated in a temperature-dependent fashion. Thus, modifications on anchor residues do not necessarilly have to affect the specificity and association kinetics of peptide binding to class I molecules but can affect the stability of the resulting class I-peptide interaction. The dissociation of peptides with modified and, more generally, suboptimal anchor residue side chains may explain the presence of empty class I molecules and free class I heavy chains at the cell surface.     

Genes encoded in the major histocompatibility complex affecting the generation of peptides for TAP transport

The B cell line 721.174 has lost the ability to present intracellular antigens to major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL). This phenotype results from a homozygous deletion in the MHC that includes the peptide transporter genes TAP1 and TAP2, and the proteasome subunits LMP2 and LMP7. Recent work has shown that such cells transfected with TAP genes load their class I molecules with endogenous peptides, and present several viral epitopes to class I-restricted CTL. These data implied that the LMP2 and LMP7 genes were not required for the presentation of most epitopes through class I molecules. By contrast, while confirming the previous reports, we have identified several epitopes that appear to require genes in the MHC in addition to the TAP for their presentation. Further analysis localizes the defect to proteolysis in the cytosol. In one case, presentation could be partially restored by re-expression of full-length LMP7. Control experiments with LMP7, from which the putative pro-region had been removed, failed to restore presentation, and this lack of effect correlated with failure of the shortened LMP7 to incorporate into the proteasome. These results suggest a role for LMP7 in the generation of a viral epitope, but leave open the possibility that additional genes within the .174 deletion are required for full restoration of antigen presentation.     

TAP1-deficient mice select a CD8+ T cell repertoire that displays both diversity and peptide specificity

Mice deficient in the gene encoding the transporter associated with antigen processing 1 (TAP1) are defective in providing major histocompatibility complex (MHC) class I molecules with cytosolic peptides. Consequently, these mice express reduced levels of MHC class I glycoproteins on the cell surface, and have reduced numbers of CD8⁺ T cells in the periphery. In the present study, we have addressed the diversity and specificity of the peripheral CD8⁺ T cell population in TAP1 -/- mice. CD8⁺ T cells were polyclonal with regard to T cell receptor (TCR) Vβ expression. Overall, Vβ usage in TAP1 -/- mice appeared to be very similar to that in wild-type mice, with significantly reduced levels of Vβ5.1/5.2-expressing CD8⁺ T cells as the only clear exception. This polyclonal population of CD8⁺ T cells readily mounted epitope-specific CTL responses against four out of five well-defined MHC class I-restricted peptides. In contrast to allospecific CTL, peptide-specific CTL from TAP1 -/- mice did not cross-react on cells expressing normal levels of H-2^b class I. The present results demonstrate that a polyclonal CD8⁺ T cell repertoire, displaying both diversity and peptide specificity, is positively selected in mice devoid of a functional peptide transporter. These observations imply that TAP-dependent peptides are not absolutely required for positive selection of a functionally diverse repertoire of CD8⁺ T cells.     

Down-regulation of class II major histocompatibility complex molecules on antigen presenting cells after interaction with helper T cells

The recognition of antigenic peptides by CD4⁺ helper T cells is demonstrated here to result in a dramatic (up to 90%) decrease in expression of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APC). The reduction is selective to the class II isotype presenting the antigen, but if affects both allelic forms of the same isotype in heterozygous APC. The observed MHC down-regulation requires a specific T cell receptor-peptide-class II interaction, a direct contact between T cell and APC, and the involvement of CD2 molecules. These findings have important implications for the regulation of immune response, self tolerance, and autoimmunity.     

TAP-dependent major histocompatibility complex class I presentation of soluble proteins using listeriolysin

Immunization of mice with mixtures of listeriolysin, a pore-forming hemolysin secreted by the pathogenic bacterium Listeria monocytogenes, together with soluble ovalbumin, nucleoprotein of influenza virus, or β-galactosidase of Escherichia coli, resulted in strong cytotoxic CD8 T cell responses to each of the respective passenger proteins in vivo. Also, the concomitant addition of either protein with listeriolysin to target cells elicited efficient sensitization of these cells which could be attributed to the pore-forming activity of listeriolysin. This response was dependent upon a functional TAP transporter and was inhibitable by brefeldin A, indicating the transfer of the soluble proteins into the cytosol and the classical major histocompatibility (MHC) class I presentation pathway. The treatment of target cells with listeriolysin under our experimental conditions did not affect cell viability and the pores generated by listeriolysin treatment were repaired within 60 min. Introduction of soluble proteins into the MHC class I presentation pathway by listeriolysin provides a powerful system to study the cytotoxic response towards intracellular pathogens and would allow for rapid screening of potential antigens in vaccine formulations.     

Major histocompatibility complex class I presentation of ovalbumin peptide 257–264 from exogenous sources: protein context influences the degree of TAP-independent presentation

Peritoneal macrophages from C57BL/6 mice process antigens from bacteria or coated on polystyrene beads for presentation by major histocompatibility complex (MHC) class I molecules. To investigate this antigen processing pathway, peritoneal macrophages from homozygous TAP1^−/− mice, which lack the transporter associated with antigen processing (TAP) and are defective in presenting endogenous antigens on MHC class I, were used. TAP1^−/− or C57BL/6 macrophages were co-incubated with either bacteria or polystyrene beads containing the 257–264 epitope from ovalbumin [OVA(257–264)], which binds the mouse class I molecule K^b. The source of the OVA(257–264) epitope was either the Crl-OVA(257–264) (Crl-OVA) fusion protein, the maltose binding protein (MBP)-Crl-OVA fusion protein, native OVA or bacterial recombinant OVA (rOVA); Crl-OVA, MBP-Crl-OVA and rOVA were each expressed in bacteria, and Crl-OVA and MBP-Crl-OVA purified from bacterial lysates and native egg OVA were coated onto polystyrene beads. The data reveal that peritoneal macrophages from C57BL/6 and TAP1^−/− mice can process bacteria expressing Crl-OVA, MBP-Crl-OVA and rOVA as well as beads coated with native OVA, purified Crl-OVA, and purified MBP-Crl-OVA and present OVA(257–264) for recognition by OVA(257–264)/K^b-specific T hybridoma cells, albeit with different relative processing efficiencies. The processing efficiency of TAP1^−/− macrophages co-incubated with bacteria or beads containing Crl-OVA or MBP-Crl-OVA was reduced approximately three to five times compared to C57BL/6 macrophages, but OVA(257–264) was presented 100 times less efficiently when the source of OVA(257–264) was full-length OVA. Chloroquine inhibition studies showed a differential requirement for acidic compartments in C57BL/6 versus TAP1^−/− macrophages, which also depended upon the source of the OVA (257–264) epitope (Crl-OVA versus full-length OVA). These data suggest that TAP1^−/− and C57BL/6 macrophages may process Crl-OVA and full-length OVA in different cellular compartments and that the protein context of the OVA(257–264) epitope influences the extent of TAP-independent processing for MHC class I presentation.     

Targeting major histocompatibility complex class II molecules to the cell surface by invariant chain allows antigen presentation upon recycling

We studied the functional consequences of targeting class II molecules to either the cell surface or to endocytic structures by expressing HLA-DR1 in human kidney cells in the presence or absence of different forms of the invariant chain (Ii). Transfectants expressing class II molecules in the absence of Ii present influenza virus efficiently and co-expression of full length Ii does not further increase antigen presentation. Chimeric Ii containing the cytoplasmic domain of the transferrin receptor (Tfr-Ii) delivers class II molecules associated with Tfr-Ii to endosomal compartments, but this does not result in efficient antigen presentation. When class II molecules are targeted to the cell surface by Ii lacking either 15 (Δ15Ii) or 23 (Δ23Ii) amino acids from the cytoplasmic domain, a fraction of free class II molecules is also observed. Whereas Δ15Ii did not affect antigen presentation by class II molecules, Δ23Ii inhibited, but did not abrogate, the response. We show that class II molecules expressed in the presence of Δ23Ii can be internalized, followed by degradation of Δ23Ii and return of free class II αβ heterodimers to the cell surface. A fraction of the resulting free class II molecules is sodium dodecyl sulfate stable, indicating that internalization and reappearance of class II molecules at the cell surface can be an alternative route for antigen presentation. In all transfectants, class II molecules were found in endocytic compartments that labeled for CD63 and resembled the multilaminar MIIC compartments found in B cell lines. Ii is not required for endosomal targeting of class II molecules. The number of class II molecules observed in the multilaminar compartments correlates with the efficiency of antigen presentation.     

Expression of class II,but not class I,major histocompatibility complex molecules is required for granuloma formation in infection with Schistosoma mansoni

Previous studies have suggested that granulomatous inflammation in schistosomiasis is mediated by CD4⁺ T helper lymphocytes sensitized to parasite egg antigens. However, CD8⁺ T cells have also frequently been associated with the immune response to schistosome eggs. To examine more precisely the role of CD4⁺ and CD8⁺ T cells in the pathology of the schistosomal infection, we used mice with targeted mutations in major histocompatibility complex (MHC) class II or class I molecules. These mutations lead, respectively, to the virtual absence of CD4⁺ and CD8⁺ T cells. The results clearly show that schistosome-infected MHC class II mutant mice failed to form granulomas around parasite eggs. In contrast, infected MHC class I mutant mice displayed characteristic granulomatous lesions that were comparable to those in wild-type control mice. Moreover, lymphoid cells from MHC class II mutant mice were unable to react to egg antigens with either proliferative or cytokine [interferon-gamma, interleukin (IL)-4, IL-10] responses; nor were they able to present egg antigens to specifically sensitized CD4⁺ T helper cells from infected syngeneic control mice. By comparison, cells from MHC class I mutant mice exercised all these functions in a manner comparable with those from wild-type controls. These observations clearly demonstrate that schistosomal egg granulomas are mediated by MHC class II-restricted CD4⁺ T helper cells. They also suggest that CD8⁺ T cells do not become sensitized to egg antigens and play little role, if any, in the pathogenesis of schistosomiasis.