Weak positive selection of transgenic T cell receptor-bearing thymocytes: importance of major histocompatibility complex class II, T cell receptor and CD4 surface molecule densities

Ig-specific T cell receptor-transgenic T cells are not deleted in the thymus and are functional in vivo

The mechanisms that induce T cell tolerance to circulating self-proteins are still controversial, and both the deletion and selection of autoreactive T cells have been observed in the thymus of transgenic mouse models. To address the question of the induction of tolerance to circulating self-constituents, a T cell receptor-transgenic mouse specific for the serum protein immunoglobulin (Ig) gamma and (IgG2ab) was generated. The choice of an allotype-specific T cell also allowed the generation of transgenic control mice not expressing the self-antigen. It was found that the transgenic T cells were not deleted in the thymus, did not become tolerant in the periphery, and regulated the function of gamma 2ab-positive B cells as shown by the lack of IgG2ab protein in the serum of the transgenic mice. In spite of this activity in vivo, the transgenic T cells did not proliferate in vitro in response to the allotype-specific peptide. Interestingly, antigen-specific T cell proliferation could be restored if the transgenic mice were previously challenged to induce IgG2ab responses. After this challenge, IgG2ab protein in the serum of the transgenic mice could be partially restored, although still remaining much lower than in control mice. In addition, there was a dramatic increase in serum IgE levels, suggesting that newly generated gamma 2ab-secreting B cells can be induced to switch to IgE in the presence of allotype-specific T cells. These results indicate that Ig-specific T cells may represent a late-acting form of T cell help for the regulation of the IgG2a-to-IgE class switch.

Preferential positive selection of T lymphocytes which express two different TCR alpha chains, an endogenous and a transgenic

A hallmark of positive selection in T-cell receptor (TCR)-transgenic mice is a strong skewing towards the CD4+ or the CD8+ subset, depending on the class II or I restriction of the TCR, respectively. However, previous experiments in TCR transgenic mice specific for an Ig light chain (lambda 2(315)/I-Ed class II molecule did not fit into this scheme because the authors observed an anomalous skewing towards CD8. In this paper the authors show that endogenous TCR alpha chains are expressed on > 90% of CD4+ and CD8+ cells in this particular transgenic strain, even on a selecting H-2d haplotype. Endogenous TCR alpha chains are first detected when double-positive thymocytes down-regulate either CD4 or CD8. Endogenous V alpha seems to influence generation of T-cell subsets because CD4+ and CD8+ cells express different frequencies of endogenous V alpha 2 and V alpha 8. In the absence of endogenous TCR alpha chains in recombination-deficient TCR-transgenic severe combined immunodeficiency (SCID) mice, a strong skewing towards CD4+ T cells is seen, but such mice are severely T-cell deficient. As an explanation for these results, the authors suggest that the transgenic TCR has a too low affinity for efficient positive selection, therefore, TCR alpha gene rearrangements proceed. Endogenous TCR alpha paired with transgenic TCR beta could bind to class I or class II molecules, enhance positive selection and thereby production of CD4+ or CD8+ cells. Most of the 'mismatched' CD8+ cells are lambda 2(315)-specific and I-Ed class II restricted, and may function as idiotype-specific suppressors of B cells. These results may help explain the origin of dual TCR alpha T cells. Furthermore, the authors suggest that T cells 'mismatched' for co-receptor/TCR MHC-specificity may be enriched among dual TCR alpha T cells.

Naive CD4+ T cells confer idiotype-specific tumor resistance in the absence of antibodies

CD4+ T cells can recognize a processed idiotypic peptide derived from the mouse lambda 2(315) immunoglobulin light chain. The idiotypic peptide is presented on the I-E(d) class II major histocompatibility complex molecule. Mice made transgenic for a lambda 2(315)-specific alpha beta T cell receptor have been demonstrated to be specifically resistant against a tumor challenge with the MOPC315 (alpha,lambda 2(315)) plasmacytoma (Lauritzsen, G. F., Weiss, S., Dembic, Z. and Bogen, B., Proc. Natl. Acad. Sci. USA 1994, 91: 5700). That study, however, did not rule out a role of either anti-Id antibodies or T cells expressing nontransgenic specificities due to expression of endogenous T cell receptor (TcR) alpha chains. Also, the role of different T cell subsets in protection was unclear. To remove these ambiguities, we have now made the transgenic mice homozygous for the scid mutation, known to inhibit both Ig and TcR gene rearrangements. Such transgenic SCID mice lack B cells and antibodies while they still have plenty of CD4+ and CD4-8- cells expressing the transgenic alpha beta T cell receptor. The number of CD8+ T cell is dramatically reduced. Even so, transgenic SCID mice are protected against a challenge with MOPC315 plasmacytoma cells. Therefore, B cells, as well as novel T cell receptor specificities created by rearrangements of endogenous alpha-chain genes, are both dispensable for effective immunosurveillance in our system. Surprisingly, we found that transgenic CD8+ and CD4-8- cells are idiotype-specific and I-E(d) restricted. However, these T cell subsets are not required for resistance because adoptive transfer experiments demonstrated that highly purified transgenic SCID CD4+ cells suffice for tumor protection.

Potent effects of low levels of MHC class II-associated invariant chain on CD4+ T cell development

Invariant chain (Ii)-negative mice exhibit defects in MHC class II assembly and transport that results in reduced levels of surface class II, altered antigen presentation, and inefficient positive selection of CD4+ T cells. Many CD4+ T cells that do mature in Ii-negative mice express a cell surface phenotype consistent with aberrant positive selection or peripheral activation. Reconstitution of these mice with low levels of either the p31 or p41 form of Ii does not restore transport of the bulk of class II or class II surface expression, but surprisingly does restore positive selection as measured by numbers and surface phenotype of CD4+ T cells. Thus, an Ii-dependent process, independent of effects on class II surface density, appears to be required for normal positive selection of CD4+ T cells.

Naive idiotype-specific CD4+ T cells and immunosurveillance of B-cell tumors

The immunosurveillance hypothesis suggests that lymphocytes can recognize tumor-specific antigens expressed by transformed cells and initiate their elimination. Immunosurveillance implies that lymphocytes of naive phenotype can home to a tumor site and become activated by tumor-specific antigens. In this study, we have employed T-cell receptor transgenic mice as a source of naive, tumor-specific T cells. The transgenic, CD4+ T cells recognize a 91- to 101-residue fragment of the lambda 2(315) immunoglobulin light chain presented by I-Ed class II molecules. Such naive, idiotype-specific, CD4+ T cells protected against tumor development of a class II negative plasmacytoma (MOPC315) and a class II positive B lymphoma (F9), which both secrete lambda 2(315) immunoglobulin. Adoptive transfer experiments demonstrated that 2 x 10(6) lymph node cells were sufficient for protection against MOPC315. Depletion of T-cell subsets indicated that transgenic CD4+ cells were indispensable for tumor resistance. However, an additional role of CD8+ T cells is not ruled out. In contrast to the resistance against the secreting MOPC315 and F9 cells, transgenic mice were not protected against B lymphoma cells (F67), which do not secrete lambda 2(315) but express a truncated lambda 2(315) chain intracellularly. The results suggest that lambda 2(315) is processed and presented by host antigen-presenting cells, which in turn activate naive, idiotype-specific T cells.

Processing and presentation of idiotypes to MHC-restricted T cells

Among the self antigens, immunoglobulins, and in particular idiotypes, are of special interest because of their extreme sequence heterogeneity and their postulated involvement in regulatory interactions in the immune system. We have therefore studied antigen processing and presentation of variable region peptides, processed idiotypes, to MHC class II molecule-restricted T cells. The immunoglobulin used has been the lambda 2(315) light chain produced by the BALB/c MOPC 315 plasmacytoma (alpha, lambda 2). The minimum length of a stimulatory synthetic idiotypic peptide comprises residues 91-101 of lambda 2(315) and is presented by the I-E(d) molecule to CD4+ T cells. T cell clones with specificity for the 91-101(lambda 2(315))/I-E(d) complex utilize a limited TCR repertoire and are of both Th1 and Th2 type. For presentation, extracellular lambda 2(315) requires endocytosis and processing, as previously described for conventional exogenous antigens. In addition, a B lymphoma cell can process and present its own endogenous lambda 2(315). This was shown by transfecting manipulated lambda 2(315) gene variants into B lymphoma cells, followed by evaluation of the APC function of the transfectants. These studies demonstrated that surface expression or secretion of lambda 2(315) is not necessary for presentation and suggested that the endoplasmic reticulum may be a processing compartment. To extend our findings to naive Id+ B cells and anti-Id T cells, we have generated lambda 2(315)-transgenic as well as TCR-transgenic mice. A model is presented for a T-B cell interaction based on presentation of processed idiotypes.

Anti-tumour activity of idiotype-specific, MHC-restricted Th1 and Th2 clones in vitro and in vivo

Idiotypes (Id) can serve as individual markers on B cells; therefore, cytotoxic Id-specific T cells may play a significant role in immunological surveillance of Id+ B-cell tumours. We have investigated the anti-tumour activity of CD4+ BALB/c Th1 and Th2 clones which recognize a processed Id of the syngeneic lambda 2(315) L chain in the context of the class II MHC molecule I-Ed. Id-specific T cells and A20/46 B lymphoma cells transfected with the lambda 2(315) gene were injected s.c. into the same site of BALB/c mice (Winn assay). The results show that both Th1 and Th2 clones can protect against tumour development. The protection was Id-specific because T cells did not influence tumour development by an A20/46 B lymphoma cell line transfected with the pSV2neo expression vector alone. In vitro studies showed that the Th1 clones were cytotoxic to lambda 2(315)-transfected B lymphoma cells; by contrast, the Th2 clone was not cytotoxic in 51Cr-release assay even though the Th2 cells inhibited the growth of lambda 2(315) B lymphoma cells. The anti-lymphoma properties of both the Th1 and Th2 clones appear to involve as yet undefined cytotoxic and growth inhibiting molecules.