Abnormal thymic development, impaired immune function and gamma delta T cell lymphomas in a TL transgenic mouse strain

Immunity against mouse thymus-leukemia antigen (TL) protects against development of lymphomas induced by a chemical carcinogen, N-butyl-N-nitrosourea

Mouse thymus-leukemia antigens (TL) are aberrantly expressed on T lymphomas in C57BL/6 (B6) and C3H/He (C3H) mice, while they are not expressed on normal T lymphocytes in these strains. When N-butyl-N-nitrosourea (NBU), a chemical carcinogen, was administered orally to B6 and C3H strains, lymphoma development was slower than in T3(b)-TL gene-transduced counterpart strains expressing TL ubiquitously as self-antigens, suggesting that anti-TL immunity may play a protective role. In addition, the development of lymphomas was slightly slower in C3H than in B6, which seems to be in accordance with the results of skin graft experiments indicating that both cellular and humoral immunities against TL were stronger in C3H than B6 mice. The interesting finding that B lymphomas derived from a T3(b)-TL transgenic strain (C3H background) expressing a very high level of TL were rejected in C3H, but not in H-2K(b) transgenic mice (C3H background), raises the possibility that TL-specific effector T cell populations are eliminated and/or energized to a certain extent by interacting with H-2K(b) molecules.

Thymus-leukemia antigen (TL) as a major histocompatibility complex (MHC) class Ib molecule and tumor-specific antigen

Mouse thymus-leukemia antigens (TL) belong to the family of major histocompatibility complex (MHC) class Ib antigens and have a unique mode of expression, i.e., in contrast to other MHC class Ib or Ia antigens, they are found restricted to the intestines in all mouse strains, but also in the thymus of certain strains (TL(+) strains). Nevertheless, a proportion of T lymphomas/leukemias in strains that do not express TL in the thymus (TL(-) strains) feature TL as a tumor antigen. TL was originally defined serologically, but subsequently we have succeeded in generating T cell receptor (TCR) and cytotoxic T lymphocytes (CTL) recognizing TL. By use of TL tetramers free from peptides and transfectants expressing various TL/H-2 chimeric molecules, we have been able to show that TL-specific CTL recognize the alpha1/alpha2 domain of TL without any additional antigen molecules. We previously reported that one of TL's functions in the thymus is positive selection of TCR CTL. Recent studies with TL tetramers revealed that they can bind to normal intestinal intraepithelial lymphocytes (iIEL) and thymocytes in a CD8-dependent, but TCR/CD3-independent manner, while their binding to TL-specific CTL is TCR/CD3- and CD8-dependent. The possible significance of these findings in relation to the roles of TL in the intestines is discussed. We have long been interested in TL as a model tumor antigen which shares characteristics with human differentiation tumor antigens, and we have demonstrated that growth of TL(+) lymphoma cells in vivo is suppressed by immunization with TL(+) skin or dendritic cells (DC) from TL transgenic mice. In addition, anti-tumor effects against TL(+) T lymphomas were obtained by adoptive transfer of TL tetramer strongly-positive TL-specific CTLs.

The binding of thymus leukemia (TL) antigen tetramers to normal intestinal intraepithelial lymphocytes and thymocytes

Thymus leukemia (TL) Ags belong to the family of nonclassical MHC class I Ags and can be recognized by both TCRalphabeta and TCRgammadelta CTL with TL, but not H-2 restriction. We previously reported that the CTL epitope is TAP independent, but the antigenic molecule(s) presented by TL has yet to be determined. In the present study, TL tetramers were prepared with T3(b)-TL and murine beta(2)-microglobulin, not including antigenic peptides, and binding specificity was studied. CTL clones against TL Ags were stained with the T3(b)-TL tetramer, and the binding shown to be CD3 and CD8 dependent. Normal lymphocytes from various origins were also studied. Surprisingly, most CD8(+) intraepithelial lymphocytes derived from the small intestines (iIEL), as well as CD8(+) and CD4(+)CD8(+) thymocytes, were stained, while only very minor populations of CD8(+) cells derived from other peripheral lymphoid tissues, such as spleen and lymph nodes, were positive. The binding of T3(b)-TL tetramers to CD8(+) iIEL and thymocytes was CD8 dependent, but CD3 independent, in contrast to that to TL-restricted CTL. These results altogether showed that TL-restricted CTL can be monitored by CD3-dependent binding of T3(b)-TL tetramers. In addition, CD3-independent T3(b)-TL tetramer binding to iIEL and thymocytes may imply that TL expressed on intestinal epithelium and cortical thymocytes has a physiological function interacting with these tetramer(+)CD8(+) T lymphocytes.

Comparison of anti-tumor responses against TL positive lymphoma induced by skin grafting and dendritic cell immunization

When the skin of Tg.Con.3-1 transgenic mice expressing the TL (thymus leukemia) antigen in most tissues is grafted on syngeneic C3H mice, it is rejected, and a cytotoxic T cell (CTL) response against the TL antigen is induced. In this study, we first demonstrated that growth of TL positive lymphoma is suppressed in mice immunized by skin grafting. Immunization with bone marrow derived dendritic cells (DCs) from Tg.Con.3-1, was also found to be associated with an anti-tumor response, but less potent than skin grafting. Relative CTL precursor frequency with DC immunization was also approximately only one third that of skin grafting. The numbers of IFN-gamma producing cells in responder CD8 and CD4 T cell populations were higher with DC immunization than with skin grafting. However, DC immunization seems to induce non-specific immune responses, as re-stimulation with TL negative C3H spleen cells resulted in induction of almost half the number observed with TL positive cells. Thus, the actual number of IFN-gamma producing cells in specific responses to TL is not necessarily larger than with skin grafting immunization. The present results altogether suggest that DC immunization is capable of inducing an anti-tumor reaction, but also possibly unwanted immune responses. In vitro monitoring of specific and non-specific responses in the immune system, thus, is of particular importance for future development of cancer immunotherapy.

The epitope detected by cytotoxic T lymphocytes against thymus leukemia (TL) antigen is TAP independent

Thymus leukemia (TL) antigens belong to the family of MHC class Ib antigens. We have shown in our previous studies that they serve as transplantation antigens, and can be recognized by both TCR alpha beta and TCR gamma delta cytotoxic T lymphocytes (CTL) with TL but not H-2 restriction. Although TL are known to be expressed TAP independently, it is unclear whether peptide loading on TL molecules is necessary for the formation of CTL epitopes. In the present study, we first showed that TL expression is beta(2)-microglobulin (beta(2)m)-dependent but TAP1 independent by flow cytometric analysis of thymocytes from beta(2)m- or TAP1-deficient mice crossed with TL transgenic mice expressing Tla(a)-3-TL on their thymocytes. Subsequently, we investigated the epitope recognized by CTL derived from C3H mice immunized with skin from a transgenic mouse expressing T3(b)-TL ubiquitously. Bulk CTL lines against TL from primary mixed lymphocyte cultures showed comparable cytotoxicity against T3(b)-TL transfectants of TAP2-deficient murine RMA-S grown at 37 degrees C to that against those grown at 25 degrees C. Furthermore, TCR alpha beta and TCR gamma delta CTL clones against TL recognized TL expressed on T3(b)-TL transfectants of RMA-S and Drosophila melanogaster cells having broad defects in peptide loading of MHC, and lysed these target cells. These results together indicate that TL-specific CTL populations primarily recognize epitopes expressed TAP independently.

Two Types of Anti-TL (Thymus Leukemia) CTL Clones with Distinct Target Specificities: Differences in Cytotoxic Mechanisms and Accessory Molecule Requirements

TCRαβ CTL clones recognizing mouse thymus leukemia (TL) Ags were established and categorized into two groups: those killing any TL+ target cells (type I) and those killing only TL+ Con A blasts (type II). Cold target inhibition assays showed that the antigenic determinant(s) recognized by type II clones are expressed not only on TL+ Con A blasts but also on other TL+ target cells. The relation of the target specificity to the killing machinery and the accessory molecules involved in cytotoxicity were therefore analyzed using four representative clones selected from each type. Of the target cells tested, Fas was only expressed on Con A blasts, indicating that Fas ligand (FasL)-dependent cytotoxicity is limited to such cells. All four type II and one of four type I clones expressed FasL on the surface, while both types contained perforin in the cytoplasm. Blocking studies using neutralizing anti-FasL mAbs and concanamycin A (CMA), a selective inhibitor of the perforin pathway, suggested that type I clones kill target cells by way of perforin, while type II clones kill TL+ Con A blasts through FasL together with perforin. For their cytotoxicity, type I CTLs require a signal through CD8, while type II require LFA-1/ICAM-1 interactions. Type II clones also need a costimulatory signal through an unknown molecule for perforin-dependent cytotoxicity. These results taken together suggest that the difference in the target specificity of anti-TL CTL clones is due to variation in the killing machineries and the dependence on accessory molecules.

Characterization of CD4+CD8+ thymocytes observed in SCID-bg mice

Previously, we reported that a strain of severe combined immunodeficient (SCID) mice, namely SCID-bg, spontaneously develop CD4+CD8+ double positive (DP) thymocytes despite their scid mutation. In the present study, we intend to further characterize the DP thymocyte population in SCID-bg mice with Southern hybridization and flow cytometry. Southern hybridization analyses of sorted DP thymocytes in SCID-bg mice showed rearrangement of T cell receptor (TCR) beta and TCR gamma gene segments, although the expression of TCR beta and gamma delta TCR molecules was absent. The phenotype of the DP thymocytes in SCID-bg mice was CD44- heat-stable antigen (HSA)+CD25-, and they did not express CD69. Interestingly, the expression of thymus leukaemia (TL) antigen was observed in the DP thymocytes of SCID-bg mice but not in their CD4-CD8- double negative (DN) fraction. Fas expression was higher and bcl-2 expression was down-regulated in the DP thymocytes as compared to the DN thymocytes in SCID-bg mice, suggesting that they are not immortalized cells having escaped from apoptosis. Taken together, these results show that the phenotype of the DP thymocytes in SCID-bg mice is similar to that of the earliest phase of the DP thymocytes in normal mice, although the expression of the molecules in the pre-TCR complex is absent.

Positive selection of gamma delta CTL by TL antigen expressed in the thymus

To elucidate the funciton of the mouse TL antigen in the thymus, we have derived two TL transgenic mouse strains by introducing Tl alpha 2-3 of A strain origin with its own promoter onto a C3H background with no expression of TL in the thymus. These transgenic mouse strains, both of which express high levels of Tla2-3-TL antigen in their thymus, were analyzed for their T cell function with emphasis on cytotoxic T lymphocyte (CTL) generation. A T cell response against TL was induced in Tg. Tla2-3-1, Tg. Tla2-3-2, and control C3H mice by skin grafts from H-2Kb/T3b transgenic mice, Tg.Con.3-1, expressing T3b-TL ubiquitously. Spleen cells from mice that had rejected the T3b-TL positive skin grafts were restimulated in vitro with Tg. Con.3-1 irradiated spleen cells. In mixed lymphocyte cultures (MLC), approximately 20% and 15% of Thy-1+ T cells derived from Tg.Tla2-3-1 and Tg.Tla2-3-2, respectively, expressed TCR gamma delta, whereas almost all those from C3H expressed TCR alpha beta. The MLC from Tg. Tla2-3-2 and C3H demonstrated high CTL activity against TL, while those from Tg. Tla2-3-1 had little or none. The generation of gamma delta CTL recognizing TL in Tg. Tla2-3-2, but not C3H mice, was confirmed by the establishment of CTL clones. A total of 14 gamma delta CTL clones were established from Tg. Tla2-3-2, whereas none were obtained from C3H. Of the 14 gamma delta CTL clones, 8 were CD8+ and 6 were CD4-CD8- double negative. The CTL activity of all these clones was TL specific and inhibited by anti-TL, but not by anti-H-2 antibodies, demonstrating that they recognize TL directly without antigen presentation by H-2. The CTL activity was blocked by antibodies to TCR gamma delta and CD3, and also by antibodies to CD 8 alpha and CD8 beta in CD8+ clones, showing that the activity was mediated by TCR gamma delta and coreceptors. The thymic origin of these gamma delta CTL clones was indicated by the expression of Thy-1 and Ly-1 (CD5), and also CD8 alpha beta heterodimers in CD8+ clones on their surfaces and by the usage of TCR V gamma 4 chains in 12 of the 14 clones. Taken together, these results suggest that Tla2-3-TL antigen expressed in the thymus engages in positive selection of a sizable population of gamma delta T cells.

Inhibition by CsA and FK506 of the in vitro proliferative response of gamma delta T cells on stimulation with anti-TCR delta monoclonal antibody

Tg.Tla'-3-1 mice are transgenic C3H/He mice with a Tlaa-3 gene derived from A mice. We demonstrated that spleen cells from Tg.Tlaa-3-1 mice, but not C3H/He mice, showed a proliferative response on stimulation with immobilized anti-TCR delta monoclonal antibody (mAb 3A10). Thus, Tg.Tlaa-3-1 mice can be useful for analysis of the function of gamma delta T cells. Using spleen cells from Tg.Tlaa-3-1 mice, we showed that cyclosporin A (CsA) and FK506 inhibited the in vitro proliferative response of gamma delta T cells on stimulation with anti-TCR delta mAb. The dose-dependent inhibitory effect of CsA and FK506 on proliferation of gamma delta T cells on stimulation with anti-TCR delta mAb was similar to that on proliferation of alpha beta T cells on stimulation with anti-TCF beta mAb. Inhibition by CsA and FK506 of proliferation of gamma delta T cells was not reversed by addition of rIL 2 (recombinant interleukin 2) during culture. The proliferative response of gamma delta T cells among spleen cells from TCR alpha beta-depleted Tg.Tlaa-3-1 mice was also inhibited by CsA and FK506, suggesting that the inhibition directly affected gamma delta T cells without mediation by alpha beta T cells.

Structure, function, and evolution of mouse TL genes, nonclassical class I genes of the major histocompatibility complex

In contrast to well-studied "classical" class I genes of the major histocompatibility complex (MHC), the biology of nonclassical class I genes remains largely unexamined. The mouse TL genes constitute one of the best defined systems among nonclassical class I genes in the T region of the MHC. To elucidate the function and the evolution of TL genes and their relationship to classical class I genes, seven TL DNA sequences, including one from a Japanese wild mouse, were examined and compared with those of several mouse and human classical class I genes. The TL genes differ from either classical class I genes or pseudogenes in the extent and pattern of nucleotide substitutions. Natural selection appears to have operated so as to preserve the function of TL, which might have been acquired in an early stage of its evolution. In a putative peptide-binding region encoded by TL genes, the rate of nonsynonymous (amino acid replacing) substitution is considerably lower than that of synonymous substitution. This conservation is completely opposite that in classical class I genes, in which the peptide-binding region has evolved to diversify amino acid sequences so as to recognize a variety of antigens. Thus, it is suggested that the function of TL antigens is distinct from that of classical class I antigens and is related to the recognition of a relatively restricted repertoire of antigens and their presentation to T-cell receptors.

TL antigen as a transplantation antigen recognized by TL-restricted cytotoxic T cells

In contrast to broadly expressed classical class I antigens of the major histocompatibility complex, structurally closely related TL antigens are expressed in a highly restricted fashion. Unlike classical class I antigens, TL antigens are not known to be targets of cytotoxic T cells or to mediate graft rejection. Whereas classical class I antigens function as antigen-presenting molecules to T cell receptors (TCR), the role of TL is yet to be defined. To elucidate the function of TL, we have derived transgenic mice expressing TL in most tissues including skin by introducing a TL gene, T3b of C57BL/6 mouse origin, driven by the H-2Kb promoter. By grafting the skin of transgenic mice, we demonstrate that TL can serve as a transplantation antigen and mediate a TCR-alpha/beta+ CD8+ cytotoxic T cell response. This T cell recognition of TL does not require antigen presentation by H-2 molecules. Furthermore, we show that C57BL/6 F1 mice develop CD8+ T cells that are cytotoxic for C57BL/6 TL+ leukemia cells, providing further support for the concept that aberrantly expressed nonmutated proteins such as TL can be recognized as tumor antigens.

Evidence for programmed cell death of self-reactive gamma delta T cell receptor-positive thymocytes

The negative selection of T cells expressing the gamma delta T cell antigen receptor (gamma delta T cells) was studied using transgenic mice expressing a gamma delta receptor with specificity for an H-2T-linked class I major histocompatibility complex molecule from H-2b mice. The potentially self-reactive gamma delta thymocytes in H-2b/d transgenic mice are larger and have lower levels of gamma delta T cell receptor expression than gamma delta thymocytes from H-2d mice. H-2b/d gamma delta thymocytes do not respond to H-2b antigen-presenting cells, and thus are inactive compared to H-2d gamma delta thymocytes. However, the H-2b/d gamma delta thymocyte population, but not the H-2d gamma delta thymocyte population, undergoes a high rate of programmed cell death when placed in overnight culture. These observations constitute the first direct evidence that self-reactive gamma delta thymocytes undergo programmed cell death. This in vitro programmed cell death of self-reactive gamma delta thymocytes may reflect the clonal deletion process that results in a depletion of gamma delta T cells in the peripheral lymphoid organs of adult H-2b/d mice. We also present evidence that self-reactive gamma delta T cells, similarly to alpha beta T cells, undergo a lesser degree of clonal deletion in neonatal mice compared to adult mice.

Extrathymic pathways of T-cell differentiation: a primitive and fundamental immune system

In addition to an intrathymic pathway of T-cell differentiation, extrathymic pathways of T-cell differentiation have recently been demonstrated to occur in multiple sites in mice. Such sites include the sinusoids of the liver, the intraepithelial region of the intestine, and the omentum of the peritoneal cavity. Although these extrathymic pathways are minimal at a young age, they become predominant with aging. Extrathymically differentiated T cells display many properties distinct from those of regular T cells of thymic origin. For instance, they consist of a considerably large proportion of gamma delta T cells as well as alpha beta T cells, contain double-negative CD4-CD8- cells and self-reactive oligoclones, constitutively express the II-2 receptor beta-chain, and have an alpha alpha homodimer of CD8 if they carry it. Cumulative evidence reveals that the extrathymic pathways comprise a primitive and fundamental immune system in the body and play a pivotal role in immune reactions under conditions of aging, bacterial infections, malignancies, autoimmune diseases, and pregnancy.