T cell selection and differential activation on structurally related HLA-DR4 ligands.

Plasticity of TCR interactions during CD4(+) T cell activation by an MHC-peptide complex accommodates variation in the peptide or MHC contact sites in which recognition of an altered ligand by the T cell can modify the T cell response. To explore the contribution of this form of TCR cross-recognition in the context of T cell selection on disease-associated HLA molecules, we have analyzed the relationship between TCR recognition of the DRB1*0401- and DRB1*0404-encoded HLA class II molecules associated with rheumatoid arthritis. Thymic reaggregation cultures demonstrated that CD4(+) T cells selected on either DRB1*0401 or DRB1*0404 could be subsequently activated by the other MHC molecule. Using HLA tetramer technology we identify hemagglutinin residue 307-319-specific T cells restricted by DRB1*0401, but activated by hemagglutinin residues 307-319, in the context of DRB1*0404. One such clone exhibits an altered cytokine profile upon activation with the alternative MHC ligand. This altered phenotype persists when both class II molecules are present. These findings directly demonstrate that T cells selected on an MHC class II molecule carry the potential for activation on altered self ligands when encountering Ags presented on a related class II molecule. In individuals heterozygous for these alleles the possibility of TCR cross-recognition could lead to an aberrant immune response.     

Differential inhibition of human antigen-specific T cell clone proliferative responses by distinct monoclonal anti-HLA-DR antibodies

The inhibition of diphtheria toxoid and varidase-specific T cell clones from a single DR 6/7 donor by eight distinct monoclonal anti-HLA-DR antibodies was tested in proliferative assays. These moAb were selected because they had been previously defined for their ability: 1) to react with Ia molecules; 2) to recognize similar or different epitopes; 3) to share or not share idiotypic specificities. Our results show a distinct inhibition pattern for each clone tested. Furthermore, the various moAb could be classified into three groups according to their inhibitory effect on T cell proliferation. These data suggest: a) an epitopic restriction by class II antigens of antigen-specific human T cell clone proliferation; and b) the recognition of functional epitopes on the human Ia-like antigens by some but not all moAb studied.     

The T cell repertoire to a multideterminant antigen. Clonal heterogeneity of the T cell response, variation between syngeneic individuals, and in vitro selection of T cell specificities.

The specificity of lysozyme-induced T cell proliferative responses by individual B10.A mice was compared by using a panel of three peptides. A surprising degree of variation in the focus of the responses was observed among individual animals, both in the newly isolated lymph node cell populations and in long term T cell lines. The responses to each determinant after hen egg-white lysozyme immunization were not equal and in examining the mice as a group some determinants tended to be dominant. However, despite each animal favoring a restricted number of determinants, the responding T cell populations were still highly heterogeneous. The data suggest that many determinants are involved in the response to the whole Ag. The role of one or more dominant determinants can be overestimated because the degree of heterogeneity in long term T cell lines appears to be less than in freshly obtained lymph node cells, indicating that a process of in vitro selection occurs. We observed that the T cells responsive to one peptide, 46-61, appeared to have a selective advantage in vitro culture.     

Nonrandom rearrangement of T cell receptor J alpha genes in bone marrow T cell differentiation cultures

TCR J alpha genes span a distance of approximately 65 kb on mouse chromosome 14. Due to the existence of 50 to 100 discrete J genes, a potential for great diversity exists within the V-J-C alpha gene products and within the ultimate repertoire of alpha beta TCR. We have prepared hybridomas from an in vitro system that supports T cell differentiation among bone marrow cells. We have examined the J alpha genes among these cells and categorized rearrangements according to their location within the J alpha locus. It was found that alpha rearrangements were always present among the hybridomas bearing beta gene rearrangements. When two bone marrow-derived alpha-bearing chromosomes could be demonstrated in these hybridomas, both were always rearranged and rearrangements on homologous chromosomes were shown to reside in similar regions of the J alpha locus. Most surprisingly, when hybridomas were categorized by the culture from which they derived, cells from the same culture (designated as a set) demonstrated a skewing of alpha rearrangements to restricted segments of J alpha genes. In one hybridoma, rearrangements on homologous chromosomes involved J alpha genes that were either identical or situated within a 1-kb segment of DNA. The skewing within sets could not be due to clonal identity between hybridomas as the beta and gamma rearrangements in all hybridomas were different. Results suggested that skewing of J alpha gene rearrangements occurred during the course of T cell development in vitro. Should the same situation occur in vivo, the number of distinct TCR J alpha sequences available for expression in early development may be far less than that predicted by gene number alone.     

Cell-cell interactions in the T cell proliferative response. I. Analysis of the cell types involved and evidence for nonspecific T cell recruitment

In the antigen-induced T cell proliferative response, it has been firmly established that antigen-specific T cell activation signals are provided by a specialized antigen-presenting cell (APC). However, the number of responding T cell populations involved has not been clearly delineated. This problem can be analyzed by plotting the logarithm of the number of cultured cells against the logarithm of the response. This yields a straight line, the slope of which indicates the minimum number of interacting cell populations that are required to give the response. In the antigen-induced T cell proliferative response, this number is 3. Based on their ability to shift the slopes of the log cell number-log response line, two of the populations were identified to be T cells. The third cell, which was present in irradiated spleens, possessed certain properties of the APC. Of the two T cells, one was antigen-specific and the other could come from normal unprimed animals. The frequency of antigen-specific proliferating T cells in primed animals was estimated to be only 1 in 1.3 x 10(3), and a large proportion of the proliferation was shown to be due to unprimed cells. Furthermore, without the need to use antigen-pulsed macrophages, this slope analysis demonstrated convincingly that the successful interaction between APCs and proliferating T lymphocytes is determined by products of the I-region immune response genes.     

Autoreactive T cell clones of MHC class II specificities are produced during responses against foreign antigens in man

Although the existence of autoreactive T cells has been widely reported in mice and in guinea pigs, a similar phenomenon is poorly documented in man. Here we report the study of three human autoreactive T cell clones isolated during immunization of HLA-DRw13 donors either against influenza A/Texas virus or against allogeneic cells. These clones are specific for autologous HLA-class II specificities either common to all HLA-DRw13 molecules or restricted to the HLA-DR products specific for the DW19 subtype of HLA-DRw13. They are also cytotoxic and they have the same specificity when tested for lytic activity or in proliferation assays. Furthermore, they are also able to help autologous B cells to polyclonally produce Ig. The possible implication of such clones in regulatory mechanisms involving HLA-class II molecules is discussed.     

Mechanisms of HLA-DR antigen expression in phytohemagglutinin-activated T cells in man. Requirement of T cell recognition of self HLA-DR antigen expressed on the surface of monocytes

Signals required for expression of HLA-DR (DR) antigen in phytohemagglutinin (PHA)-activated human peripheral blood T cells were examined. T cells were purified by a four-step procedure, which included depletion of glass-adherent cells, 53% Percoll gradient centrifugation, nylon wool column passage, and treatment with mouse monoclonal antibodies directed to human HLA-DR antigen and Leu M1 antigen plus complement. Purified T cells responded poorly to PHA but with the combination stimuli of PHA and recombinant human interleukin 2 (rIL-2), resting T cells proliferated as well as T cells cultured with 10% monocytes and PHA. But well proliferated T cells in the absence of monocytes expressed very poor DR antigen after 7 to 8 days of culture. DR expression of T cells was restored by the addition of 10% monocytes. Allogeneic monocytes also helped proliferative responses of PHA-activated T cells but did not help the expression of DR antigen. These results suggested that signals required for T cell proliferation (PHA and rIL-2) were not sufficient for DR expression in this system and further monocytes were essentially required in a HLA-restricted manner. In the next experiment, we examined the role of membrane molecules in monocytes for transmission of signals that induce activated T cells to express DR antigen. Autologous monocytes were fixed with 1% paraformaldehyde and added to T cells in the presence of PHA and rIL-2. Fixed monocytes could help DR antigen expression of PHA-activated T cells as well as viable monocytes. But when fixed monocytes were pretreated with anti-HLA-DR monoclonal antibody, they could not help DR expression of T cells any longer. These results suggested that for the expression of DR antigen, PHA-activated T cells had to first recognize self DR antigen expressed on the surface of monocytes before proliferation occurred.     

Regulation of immune responses by T cell subsets. Role of helper and suppressor T cells in the development and expression of MHC-restricted antibody responses to L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT) by (responder X responder)F1 spleen cells

The roles of helper and suppressor T cells in the development and expression of antibody responses to GAT were studied in (responder X responder)F1 mice immunized with parental GAT-M phi. Spleen cells from (B10 X B10.D2)F1 mice primed in vivo with B10 or B10.D2 GAT-M phi developed secondary in vitro plaque-forming cell (PFC) responses only when stimulated by GAT-M phi syngeneic with the GAT-M phi used for in vivo priming. By contrast, virgin F1 spleen cells developed comparable primary PFC responses to both parental GAT-M phi Co-culture of T cells from (B10 X B10.D2)F1 mice primed in vivo by B10 GAT-M phi with virgin (B10 X B10.D2)F1 spleen cells demonstrated the presence of suppressor cells that inhibited the primary response of virgin spleen cells stimulated by B10.D2 GAT-M phi. Spleen cells from (B10 X B10.D2)F1 mice primed in vivo with B10.D2 GAT-M phi had suppressor T cells that suppressed primary responses stimulated by B10 GAT-M phi. The suppressor T cell mechanism was composed of at least two regulatory T cell subsets. Suppressor-inducer T cells were Lyt-2-, I-J+ and must be derived from immune spleen cells. Suppressor-effector T cells can be derived from virgin or immune spleens and were Lyt-2+ cells. When the suppressor mechanism was disabled by treatment with 1000 rad gamma irradiation or removal of Lyt-2+ cells, Lyt-2-helper T cells from (B10 X B10.D2)F1 mice primed with B10 GAT-M phi provided radioresistant help to virgin F1 B cells stimulated by B10 but not B10.D2 GAT-M phi. Suppressor inducer Lyt-2-,I-J+ cells from B10 GAT-M phi-primed (B10 X B10.D2)F1 mice were separated from the primed Lyt-2-,I-J-helper T cells. In the presence of Lyt-2+ suppressor effector cells, the Lyt-2-,I-J+ suppressor-inducer suppressed the primary response of virgin spleen or virgin T plus B cells stimulated by both B10 and B10.D2 GAT-M phi. Therefore, suppressor T cells were able to suppress primary but not secondary GAT-specific PFC responses stimulated by either parental GAT-M phi. These results showed that immunization of (responder X responder)F1 mice with parental GAT-M phi results in the development of antigen-specific helper and suppressor T cells. The primed helper T cells were radioresistant and were genetically restricted to interact with GAT in association with the major histocompatibility complex antigens of the M phi used for in vivo priming.(ABSTRACT TRUNCATED AT 400 WORDS)     

Nonmethylated CG motifs packaged into virus-like particles induce protective cytotoxic T cell responses in the absence of systemic side effects

DNA rich in nonmethylated CG motifs (CpGs) greatly facilitates induction of immune responses against coadministered Ags. CpGs are therefore among the most promising adjuvants known to date. Nevertheless, CpGs are characterized by two drawbacks. They have unfavorable pharmacokinetics and may exhibit systemic side effects, including splenomegaly. We show in this study that packaging CpGs into virus-like particles (VLPs) derived from the hepatitis B core Ag or the bacteriophage Qbeta is a simple and attractive method to reduce these two problems. CpGs packaged into VLPs are resistant to DNase I digestion, enhancing their stability. In addition, and in contrast to free CpGs, packaging CpGs prevents splenomegaly in mice, without affecting their immunostimulatory capacity. In fact, vaccination with CpG-loaded VLPs was able to induce high frequencies of peptide-specific CD8(+) T cells (4-14%), protected from infection with recombinant vaccinia viruses, and eradicated established solid fibrosarcoma tumors. Thus, packaging CpGs into VLPs improves both their immunogenicity and pharmacodynamics.     

T cell responses against microsatellite instability-induced frameshift peptides and influence of regulatory T cells in colorectal cancer

High-level microsatellite-unstable (MSI-H) colorectal carcinomas (CRC) represent a distinct subtype of tumors commonly characterized by dense infiltration with cytotoxic T cells, most likely due to expression of MSI-H-related frameshift peptides (FSP). The contribution of FSP and classical antigens like MUC1 and CEA to the cellular immune response against MSI-H CRC had not been analyzed so far. We analyzed tumor-infiltrating and peripheral T cells from MSI-H (n = 4 and n = 14, respectively) and microsatellite-stable (MSS) tumor patients (n = 26 and n = 17) using interferon gamma ELISpot assays. Responses against 4 FSP antigens and peptides derived from MUC1 to CEA were compared with and without depletion of regulatory T cells, and the results were related to the presence of the respective antigens in tumor tissue. Preexisting FSP-specific T cell responses were detected in all (4 out of 4) tumor-infiltrating and in the majority (10 out of 14) of peripheral T cell samples from MSI-H CRC patients, but rarely observed in MSS CRC patients. Preexisting T cell responses in MSI-H CRC patients were significantly more frequently directed against FSP tested in the present study than against peptides derived from classical antigens MUC1 or CEA (p = 0.049). Depletion of regulatory T cells increased the frequency of effector T cell responses specific for MUC1/CEA-derived peptides and, to a lesser extent, T cell responses specific for FSP. Our data suggest that the analyzed FSP may represent an immunologically relevant pool of antigens capable of eliciting antitumoral effector T cell responses.     

Induction of T cell responses and recruitment of an inflammatory dendritic cell subset following tumor immunotherapy with Mycobacterium smegmatis

Mycobacteria and their cell wall components have been used with varying degrees of success to treat tumors, and Mycobacterium bovis BCG remains in use as a standard treatment for superficial bladder cancer. Mycobacterial immunotherapy is very effective in eliciting local immune responses against solid tumors when administered topically; however, its effectiveness in eliciting adaptive immune responses has been variable. Using a subcutaneous mouse thymoma model, we investigated whether immunotherapy with Mycobacterium smegmatis, a fast-growing mycobacterium of low pathogenicity, induces a systemic adaptive immune response. We found that M. smegmatis delivered adjacent to the tumor site elicited a systemic anti-tumor immune response that was primarily mediated by CD8⁺ T cells. Of note, we identified a CD11c⁺CD40^intCD11b^hiGr-1⁺ inflammatory DC population in the tumor-draining lymph nodes that was found only in mice treated with M. smegmatis. Our data suggest that, rather than rescuing the function of the DC already present in the tumor and/or tumor-draining lymph node, M. smegmatis treatment may promote anti-tumor immune responses by inducing the involvement of a new population of inflammatory cells with intact function.     

Sequence diversity, natural selection and linkage disequilibrium in the human T cell receptor alpha/delta locus

T cell receptors (TR), through their interaction with the major histocompatibility complex, play a central role in immune responsiveness and potentially immune-related disorders. We resequenced all 57 variable (V) genes in the human T cell receptor alpha and delta (TRA/TRD) locus in 40 individuals of Northern European, Mexican, African-American and Chinese descent. Two hundred and eighty-four single nucleotide polymorphisms (SNPs) were identified. The distribution of SNPs between V genes was heterogeneous, with an average of five SNPs per gene and a range of zero to 15. We describe the patterns of linkage disequilibrium for these newly discovered SNPs and compare these patterns with other emerging large-scale datasets (e.g. Perlegen and HapMap projects) to place our findings into a framework for future analysis of genotype–phenotype associations across this locus. Furthermore, we explore signatures of natural selection across V genes. We find evidence of strong directional selection at this locus as evidenced by unusually high values of F _st Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Mapping of distinct serologic and T cell recognition epitopes on an HLA-DR beta-chain.

A new DR beta-chain allele is defined that is identical to the previously described DR6b molecule except for the first hyperpolymorphic region, where the new allele displays the same polymorphisms found on DR8 and DR12 genes. Two distinct epitopes have been mapped on this new allele. The polymorphism in common with DRw8 and DRw12 is recognized by mAb GS313-9D11. However, alloreactive T cell clones specific for DR6b cells (Dw9) recognize this allele, whereas Dw8-specific T cell clones do not. The mAb determinant maps to the first beta-sheet and probably involves a polymorphic residue lying outside the helix. The binding of mAb 9D11 to this region does not interfere with TCR binding. Alloreactive T cell recognition is associated with polymorphisms located predominantly on the alpha-helical portion of the molecule.     

Alveolar macrophages in pulmonary immune responses. I. Role in the initiation of primary immune responses and in the selective recruitment of T lymphocytes to the lung

Antigen inoculated intratracheally (IT) into animals can induce primary immune responses and selectively recruit specific T cells to the lung. In the current study, the role of alveolar macrophages (AM) in these two responses was investigated. Antigen-pulsed bronchoalveolar cells (BAC) inoculated IT into guinea pigs generated a population of immune T cells that proliferated in vitro on reexposure to antigen-pulsed macrophages (M?). The possibility that antigen-pulsed donor BAC shed antigen that was subsequently processed and presented by host M? was ruled out by genetic experiments. Thus, peritoneal exudate lymphocytes (PEL) from (2 X 13)F1 guinea pigs primed with antigen-pulsed BAC from strain 2 animals responded preferentially to antigen-pulsed strain 2 M? rather than to antigen-pulsed strain 13 M?. In a second set of studies, antigen-pulsed BAC inoculated IT into guinea pigs selectively recruited antigen-specific T cells to the lung. Genetic experiments verified that inoculated BAC were the source of the antigen-presenting cells responsible for selective recruitment. Thus, antigen-pulsed strain 2 BAC inoculated IT recruited a greater proportion of (2 X 13)F1 T cells that recognized antigen in the context of strain 2 M? than F1 T cells that recognized antigen on strain 13 M?. Taken together, these studies suggest that AM contribute to the regulation of pulmonary immunity by both inducing T lymphocyte immunity and selectively recruiting specific T cells to the lung.     

T cell recruitment from the thymus to the spleen in tumor-bearing mice

Summary After inoculation of tumor cells (methylcholanthrene-induced sarcoma), the number of Thy 1⁺ cells and PNA (peanut agglutinin) binding cells, which were shown to be different subpopulations were increased in the spleen of thymus-intact mice, in contrast this increase was not observed in adult thymectomized mice. In experiments performed concurrently with splenic cell analysis, we found that the plasma PGE₂ levels declined in parallel with the tumor growth. Prevention of such a decline of plasma PGE₂ level by replenishment with exogenous PGE₂ inhibited the splenic cell increase in tumor bearers. In the tumorbearing mice, cell traffic systems from the thymus to the periphery was ascertained by injecting fluorescein diacetate (FDA) into the thymus and observing fluorescein positive cells in the periphery. We suggest that increased recruitment of thymic cells to the periphery may be mediated by PGE₂ in the presence of a tumor.Abbreviations PNA⁺ cells, peanut agglutinin binding cells - Ig⁺ cells, surface IgM positive cells - Thy 1⁺ Thy 1.2 antigen positive cells - PGE₂ prostaglandin E₂     

Distinct T cell specificities are induced with the authentic versus recombinant Plasmodium berghei circumsporozoite protein.

Plasmodium berghei sporozoite (SPZ)-immune lymph node (LN) cells obtained from mice of different H-2 haplotypes were analyzed for the presence of circumsporozoite (CS) protein-reactive T cells in proliferative assays. Although lymphocytes from each strain responded in vitro to the priming Ag and to the soluble rCS protein, they did not respond to CS protein synthetic peptides. Parallel analysis of rCS protein-primed LN cells revealed that the two Ag are unequal in generating T cell specificities: although SPZ priming did not induce CS protein peptide-reactive T cells, priming with rCS protein did. Not being privy to the processing and presentation of SPZ Ag, we postulated that a different order of processing of the authentic, i.e., SPZ-associated CS protein vs soluble rCS protein might be responsible for the generation of different T cell specificities. Accordingly, authentic CS protein might not be processed by APC, or the processed fragments might obscure the recognition of smaller peptide fragments. Therefore, we subjected the SPZ to three cycles of a freeze/thaw procedure and used the denatured SPZ preparation for priming. We observed that contrary to priming with the authentic SPZ, denatured SPZ generated T cells reactive to some of the CS protein synthetic peptides. The hypothesis that each form of the SPZ Ag is subject to a unique Ag processing was also confirmed in experiments demonstrating a lack of recognition of the authentic CS protein by rCS protein-primed LN cells. Hence, the evidence presented in this work that complex protozoan Ag, such as Plasmodia, might present different requirements for Ag-specific T cell induction/activation not only enhances the basic understanding of the immune system, but is essential for the development of antimalaria vaccine(s). In addition, these observations support the hypothesis that the molecular context of the priming Ag influences the outcome of T cell specificities, by providing evidence that the authentic CS protein induces a T cell repertoire that is distinct from that induced by the rCS protein.     

Mycobacterium leprae-specific T cells from a tuberculoid leprosy patient suppress HLA-DR3-restricted T cell responses to an immunodominant epitope on 65-kDa hsp of mycobacteria.

The polar tuberculoid type (TT) of leprosy, characterized by high T cell reactivity to Mycobacterium leprae, is associated with HLA-DR3. Surprisingly, DR3-restricted low T cell responsiveness to M. leprae was found in HLA-DR3-positive TT leprosy patients. This low responsiveness was specifically induced by M. leprae but not by M. tuberculosis and was seen only in patients and not in healthy controls. We studied this patient-specific, M. leprae-induced, DR3-restricted low T cell responsiveness in depth in one representative HLA-DR3-positive TT leprosy patient by using T cell clones. From this patient two types of T cell clones were obtained: one type was cross-reactive with M. tuberculosis and recognized an immunodominant epitope (amino acids 3 to 13) on the 65-kDa heat shock protein (hsp) the other type was M. leprae specific and reacted to a protein other than the 65-kDa one. To examine whether these M. leprae-specific T cell clones were responsible for the DR3-restricted low responsiveness to M. leprae, we tested them for the ability to suppress the proliferation of the DR3-restricted, 65-kDa, hsp-reactive clones. The DR3-restricted, M. leprae-specific T cells completely suppressed the proliferative responses of DR3-restricted, cross-reactive T cell clones to the 65-kDa hsp from the same patient as well as from other individuals. Also, DR3-restricted responses to an irrelevant Ag were suppressed by the M. leprae-specific T cell clones. However, no suppression of non-DR3-restricted T cell responses was seen. Although the mechanism must still be elucidated, this M. leprae-induced, DR3-restricted immunosuppression may at least partly explain the observed DR3-associated low T cell responsiveness in TT leprosy patients.     

Immunoregulatory pathways in adult responder mice. II. Regulation of delayed-type hypersensitivity responses by GAT-specific suppressor factors present in GAT-tolerant adult responder mice

We studied the effects of T cell extracts from adult responder BALB/c mice tolerized with poly(Glu60Ala30Tyr10) (GAT)-coupled syngeneic spleen cells (GAT-SP) on delayed-type hypersensitivity (DTH), T cell-proliferative (Tprlf), and plaque-forming cell (PFC) responses. Adult responder mice injected i.v. with GAT-SP develop Lyt-1-2+ suppressor T cells (Ts), which suppress the induction of GAT-specific DTH and PFC, but not Tprlf responses. Sonicates from these Ts contain an afferent-acting, soluble factor(s) (GAT-TsFdh) that specifically suppresses the same responses as the intact Ts (i.e., DTH and PFC, but not Tprlf). Immunosorbent chromatography studies were employed to determine the molecular nature of the suppressive material active on both cellular and humoral responses. In both assay systems, GAT-TsFdh was found to bear determinants encoded by the I subregion of the H-2 complex and a receptor(s) for GAT. BALB/c-derived GAT-TsFdh suppressed the induction of GAT DTH in syngeneic BALB/c and H-2-compatible B10.D2, but not in allogeneic C57BL/6 or CBA/Cum, suggesting a possible H-2 restriction in the suppression. It was also shown that one target of functional regulation by GAT-TsFdh is the T helper cell for DTH responses (DTH-Th). The results suggest that similar Ts and TsF regulate humoral and cell-mediated responses, perhaps by affecting a target common to both pathways (e.g., the T helper cell). The resistance of Tprlf responses to suppression by GAT-TsFdh indicates that the effector DTH-Th target is not a major component of the proliferative response. These data are discussed with respect to GAT-specific TsF-regulating PFC responses, which have been identified in nonresponders and in responders tolerized as neonates with GAT.     

Involvement of the SHP-1 tyrosine phosphatase in regulation of T cell selection.

The selection events shaping T cell development in the thymus represent the outcome of TCR-driven intracellular signaling cascades evoked by Ag receptor interaction with cognate ligand. In view of data indicating TCR-evoked thymocyte proliferation to be negatively modulated by the SHP-1 tyrosine phosphatase, a potential role for SHP-1 in regulating selection processes was investigated by analysis of T cell development in H-Y TCR transgenic mice rendered SHP-1 deficient by introduction of the viable motheaten mutation or a dominant negative SHP-1-encoding transgene. Characterization of thymocyte and peripheral T cell populations in H-Y TCR-viable motheaten mice revealed TCR-evoked proliferation as well as the positive and negative selection of H-Y-specific thymocytes to be enhanced in these mice, thus implicating SHP-1 in the negative regulation of each of these processes. T cell selection processes were also augmented in H-Y TCR mice carrying a transgene driving lymphoid-restricted expression of a catalytically inert, dominant-negative form of SHP-1. SHP-1-negative effects on thymocyte TCR signaling were not influenced by co-cross-linking of the CD28 costimulatory and/or CTLA-4 inhibitory receptors and appear, accordingly, to be realized independently of these comodulators. These observations indicate that SHP-1 raises the signaling threshold required for both positive and negative selection and reveal the inhibitory effects of SHP-1 on TCR signaling to be cell autonomous. The demonstrated capacity for SHP-1 to inhibit TCR-evoked proliferation and selection indicate SHP-1 modulatory effects on the magnitude of TCR-generated signal to be a key factor in determining the cellular consequences of TCR-ligand interaction.