Eddy Bruyns

Integration of receptor-mediated signals in T cells by transmembrane adaptor proteins

Abstract Our knowledge of the molecular events leading to T-cell activation has not yet fully explained how intracellular effector molecules are recruited to the plasma membrane after engagement of the TCR by antigen–MHC. The identification of transmembrane adaptor proteins might help to elucidate the mechanisms involved.

The Transmembrane Adapter Protein SIT Regulates Thymic Development and Peripheral T-Cell Functions

ABSTRACT SIT is a transmembrane adapter protein that modulates signals emanating from the T-cell receptor (TCR). Here, we have used gene-targeted mice to assess the role of SIT for T-cell development and peripheral T-cell functions. SIT−/− double-positive thymocytes show an upregulation of the activation markers CD5 and CD69, suggesting that SIT negatively regulates TCR-mediated signals at the CD4+ CD8+ stage of thymic development. This assumption is further supported by the observation that in female H-Y TCR transgenic mice, positive selection is enhanced and even converted to negative selection. Similarly, mature peripheral T cells are hyperresponsive towards TCR-mediated stimuli and produce larger amounts of T-helper 1 (TH1) cytokines, and SIT-deficient mice show an increased susceptibility to develop experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. These results demonstrate that SIT is a critical negative regulator of TCR-mediated signaling and finely tunes the signals required for thymic selection and peripheral T-cell activation.

Normal T-Cell Development and Immune Functions in TRIM-Deficient Mice

ABSTRACT The transmembrane adaptor molecule TRIM is strongly expressed within thymus and in peripheral CD4+ T cells. Previous studies suggested that TRIM is an integral component of the T-cell receptor (TCR)/CD3 complex and might be involved in regulating TCR cycling. To elucidate the in vivo function of TRIM, we generated TRIM-deficient mice by homologous recombination. TRIM−/− mice develop normally and are healthy and fertile. However, the animals show a mild reduction in body weight that appears to be due to a decrease in the size and/or cellularity of many organs. The morphology and anatomy of nonlymphoid as well as primary and secondary lymphoid organs is normal. The frequency of thymocyte and peripheral T-cell subsets does not differ from control littermates. In addition, a detailed analysis of lymphocyte development revealed that TRIM is not required for either positive or negative selection. Although TRIM−/− CD4+ T cells showed an augmented phosphorylation of the serine/threonine kinase Akt, the in vitro characterization of peripheral T cells indicated that proliferation, survival, activation-induced cell death, migration, adhesion, TCR internalization and recycling, TCR-mediated calcium fluxes, tyrosine phosphorylation, and mitogen-activated protein family kinase activation are not affected in the absence of TRIM. Similarly, the in vivo immune response to T-dependent and T-independent antigens as well as the clinical course of experimental autoimmune encephalomyelitis, a complex Th1-mediated autoimmune model, is comparable to that of wild-type animals. Collectively, these results demonstrate that TRIM is dispensable for T-cell development and peripheral immune functions. The lack of an evident phenotype could indicate that TRIM shares redundant functions with other transmembrane adaptors involved in regulating the immune response.

Biochemical and functional analysis of mice deficient in expression of the CD45-associated phosphoprotein LPAP.

The role of the CD45‐associated phosphoprotein (LPAP / CD45‐AP) during an immune response remains unclear. To understand better the function of LPAP we generated LPAP‐deficient mice by disrupting exon 2 of the LPAP gene. LPAP‐null mice were healthy and did not show gross abnormalities compared to their wild‐type littermates. However, immunofluorescence analysis of T and B lymphocytes revealed a reduced expression of CD45, which did not affect a particular subpopulation. In contrast to a recent report (Matsuda et al., J. Exp. Med. 1998. 187: 1863 – 1870) we neither observed significant alterations of the assembly of the CD45 / lck‐complex nor of polyclonal T‐cell responses. However, lymphnodes from LPAP‐null mice showed increased cellularity, which could indicate that expression of LPAP might be required to prevent expansion of lymphocytes in particular lymphatic organs rather than potentiating immune responses.