Correlation of Qa-1 determinants defined by antisera and by cytotoxic T lymphocytes

Anti-STAT6 CTL activity in Stat6 (-/-) mice: A cautionary tale

The generation of germline gene mutations in mice has been an invaluable tool for experimental biology. However, studying immune responses that develop in the absence of a specific protein that could alter thymic selection complicates experimental interpretations. We observed that CD8⁺ T cells from Stat6^−/− mice displayed “autoreactivity” to STAT6-expressing cells, associated with specific STAT6 peptides binding to MHC class I molecules. These results suggest caution in interpreting experiments where STAT6-expressing cells are transferred into Stat6^−/− mice, or where adoptive transfer of Stat6^−/− lymphocytes is performed. Our results further highlight additional considerations when studying immune responses involving cell transfer into gene-deficient mice.

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.

A fail-safe mechanism for maintaining self-tolerance

Using cytotoxic T lymphocyte (CTL) responses to the class I histocompatibility antigen Qa1 and to the minor histocompatibility antigen H-Y, we show that the immune system maintains a peripheral screening process that is able to tolerize a wide variety of potentially autoimmune CTL. The critical factor is the presence or absence of specific T helper cells. If T help is available, CTL precursors that recognize antigen are activated. In the absence of help, they are tolerized. Thus, T helper cells are guardians of peripheral tolerance in CTL.

The thymus leukemia antigen binds human and mouse CD8

The thymus leukemia antigen (TLA) is a class Ib, or 'nonclassical' class I molecule, one of several encoded within the Tla locus of the mouse major histocompatibility complex (MHC). It structurally resembles the H-2K, D, and L class I transplantation antigens, which present processed peptides to cytotoxic T lymphocytes (CTLs). Although their function(s) are unknown, there has been recent speculation concerning the possibility that class Ib molecules may present antigens to T cells that express gamma delta T cell antigen receptors (TCRs). In this report, using both a cell-cell adhesion assay and adhesion of T lymphocyte clones to purified plate-bound TLA, we provide evidence that TLA can bind to both human and mouse CD8. We also show that a chimeric class I molecule containing the peptide antigen binding site of Ld and the alpha 3 domain, transmembrane, and cytoplasmic segments of TLA, can support a CD8-dependent immune response by CTLs. These results demonstrate for the first time binding of a class Ib molecule to CD8 with a functional outcome, as is observed for the class I transplantation antigens. The capacity to interact with CD8 has been conserved despite the extensive sequence divergence of TLA in the peptide antigen binding site, suggesting this interaction is highly significant. TLA is expressed by epithelial cells in the mouse small intestine. As these epithelial cells are in close contact with intestinal intraepithelial lymphocytes that are nearly all CD8+, and many of which express the gamma delta TCR, the data are consistent with the hypothesis that TLA is involved in antigen presentation, perhaps to gamma delta-positive lymphocytes in this site.

The TL region gene 37 encodes a Qa-1 antigen

Of all the biochemically defined mouse MHC class I molecules, the Qa-1 antigens are the only ones for which a gene has not been identified. Recent evidence has suggested that Qa-1 antigens are functional class I molecules and can function as restriction elements for gamma/delta T cells. We have examined the relationship between Qa-1 and the product of gene 37, a presumed novel class I antigen encoded within the TL region. Immunoprecipitation and polyacrylamide gel electrophoresis analysis of the molecules reactive with anti-Qa-1 and anti-37 sera show that the Qa-1 molecule of Qa-1b (Qa-1.2) mouse strains is identical to the product of gene 37 on the basis of molecular weight, pI, and strain distribution. Immunodepletion, biosynthetic labeling, and tunicamycin treatment confirm that the protein encoded by gene 37 in Qa-1b mice is Qa-1.2. In contrast, the anti-37 serum was unable to recognize the Qa-1 molecule in Qa-1a strains. Given the fact that the only allele to gene 37 thus far identified in a Qa-1a strain (A/J) has a termination codon in the alpha 3 domain, our data lead us to conclude that the Qa-1 molecule expressed in Qa-1a mice is not a true allelic product of the gene 37 encoded antigen of Qa-1b mouse strains.