Mls determinants recognized by T cells

The Thymus in Chagas Disease: Molecular Interactions Involved in Abnormal T-Cell Migration and Differentiation

Chagas disease, caused by the protozoan parasite T. cruzi, is a prevalent parasitic disease in Latin America. Presently, it is spreading around the world by human migration, thus representing a new global health issue. Chronically infected individuals reveal a dissimilar disease progression: while nearly 60% remain without apparent disease for life, 30% develop life-threatening pathologies, such as chronic chagasic cardiomyopathy (CCC) or megaviscerae. Inflammation driven by parasite persistence seems to be involved in the pathophysiology of the disease. However, there is also evidence of the occurrence of autoimmune events, mainly caused by molecular mimicry and bystander activation. In experimental models of disease, is well-established that T. cruzi infects the thymus and causes locally profound structural and functional alterations. The hallmark is a massive loss of CD4⁺CD8⁺ double positive (DP) thymocytes, mainly triggered by increased levels of glucocorticoids, although other mechanisms seem to act simultaneously. Thymic epithelial cells (TEC) exhibited an increase in extracellular matrix deposition, which are related to thymocyte migratory alterations. Moreover, medullary TEC showed a decreased expression of AIRE and altered expression of microRNAs, which might be linked to a disrupted negative selection of the T-cell repertoire. Also, almost all stages of thymocyte development are altered, including an abnormal output of CD4⁻CD8⁻ double negative (DN) and DP immature and mature cells, many of them carrying prohibited TCR-Vβ segments. Evidence has shown that DN and DP cells with an activated phenotype can be tracked in the blood of humans with chronic Chagas disease and also in the secondary lymphoid organs and heart of infected mice, raising new questions about the relevance of these populations in the pathogenesis of Chagas disease and their possible link with thymic alterations and an immunoendocrine imbalance. Here, we discuss diverse molecular mechanisms underlying thymic abnormalities occurring during T. cruzi infection and their link with CCC, which may contribute to the design of innovative strategies to control Chagas disease pathology.

The superantigen Pseudomonas exotoxin A requires additional functions from accessory cells for T lymphocyte proliferation

We have examined the functions required of accessory cells (AC) for murine thymocyte proliferation induced by Pseudomonas exotoxin A (PE) and have compared these functions to those required of a known superantigen, staphylococcal enterotoxin B (SEB). We demonstrate that PE, like SEB, preferentially stimulates PNA+ thymocytes expressing a specific V beta element within the T cell receptor. However, PE requires functions from AC that are distinct from those required by SEB. AC treated with paraformaldehyde (PCHO) prior to stimulation supported thymocyte proliferation induced by SEB but not PE. However, when AC were treated with PCHO subsequent to stimulation with PE, thymocyte proliferation was observed, which suggests that PE requires antigen processing in addition to presentation. Furthermore, treatment of AC with lysosomotropic agents abrogated thymocyte proliferation induced by PE but not SEB. Antibodies to MHC class II molecules inhibited thymocyte proliferation induced by both PE and SEB. In addition, we observed that interleukin 1 alpha (IL-1 alpha) participated in the proliferation of thymocytes induced by PE but not SEB. Thus, our data indicate that PE is a unique microbial superantigen that requires additional AC functions for T lymphocyte proliferation.

Mls allo-determinants are recognized in an MHC class II antigen-dependent but unrestricted fashion by a discrete set of T cells

Blocking studies carried out with anti-H-2 class II antigen antibodies show that H-2 class II molecules are intimately involved in the recognition of Mlsa determinants by unprimed, specifically responsive T cells. The blocking of the anti-Mlsa response by these antibodies were not due to inhibition of IL-1 production by H-2 class II antigen positive macrophages. A strain analysis indicates that the response to Mlsa is regulated by H-2-linked genes and that this effect is exerted at the level of the stimulator cells; however, the response to Mls epitopes does not appear to be H-2 restricted. Finally, the ability of spleen cells from Mls-incompatible mice to induce a state of Mls-specific, clonal deletion type tolerance by neonatal injection, shows that Mls determinants exist in qualitatively allelic forms and suggests that these determinants are recognized by specific clones of T cells.