Kraj Lab People 2016

The laboratory of Dr. Kraj studies the development and function of CD4⁺ T lymphocytes and mechanisms of immunological tolerance. A number of cellular, molecular and bioinformatics tools are used. The goal of this research is to uncover how tolerance mechanisms could be modulated to augment immune responses in cancer and control these responses in autoimmunity.

The discovery of regulatory T cells (T_R) expressing transcription factor Foxp3 brought a new paradigm that immune tolerance is established and maintained by active suppression. T_R cells maintain immunological tolerance; sustain homeostasis of the immune system and control antigenic responses of effector T cells. However, these cells are also responsible for suppressing immune responses to tumor cells allowing cancer to grow. Dr. Kraj investigates how thymic development of T_R cells is regulated, what factors control maintenance and generation of T_R cells in the periphery, and what is the mechanism of T_R suppressor function.

To discriminate T_R cells, a reporter mouse was produced that expresses green fluorescent protein controlled by the Foxp3 regulatory sequences and was used to show that the peripheral population of T_R cells is heterogeneous and T_R subsets differentially respond to antigenic stimulation with conventional and tumor-associated antigens. Comparison of the global gene expression profiles of conventional CD4⁺ T cells and T_R cells identified genes that are T_R cell-specific and that could be targeted to modulate T_R cell function.

One of the identified genes, which controls T_R cell suppressor function, is connexin 43, a gap junction protein. By deleting connexin 43 in T cells, Dr. Kraj’s laboratory found that it controls Foxp3 expression and thymic generation of T_R cells. Recently, we found that compromised suppression mediated by T_R cells was associated with decreased expression of Cx43 in NOD mice, a mouse model of diabetes. The suppressor function of T_R cells can be corrected in vitro, even in T cells isolated from mice with advanced diabetes, by a novel reagent, which facilitates gap junction formation.

The laboratory is also investigating how Bone Morphogenic Protein Receptor 1α (BMPR1α) signaling controls functions of conventional and T_R cells. BMPR1α (Alk-3), expressed by activated effector and T_R cells modulates functions of both cell types. Bone Morphogenic Proteins (BMPs) belong to the TGF-β family of cytokines, which also includes TGF-β and activins. BMPs play crucial roles in embryonic development, tissue differentiation and homeostasis, and development of cancer. It was demonstrated that BMPs and activins synergize with TGF-β to regulate thymic T cell development, and maintain T_R cells and peripheral tolerance, but the precise mechanism of their function is not known. BMPR1α ligands, BMPs, are produced by multiple tumors and we hypothesized that BMPR1α augments suppressor function of existing T_R cells, promotes conversion of activated conventional CD4⁺ T cells into adaptive T_R cells and modulates balance between the subsets of effector CD4⁺ T cells. We have recently shown BMPR1α gene deficient CD4⁺ T cells. Mice where BMPR1α is deleted in T cells (BMPR1α^T- mice) had a decreased proportion of T_R cells and activated T cells did not produce adaptive T_R cells but preferentially to develop into pro-inflammatory Th17 cells. Moreover, B16 melanoma tumors grew smaller in BMPR1α^T- mice and tumors had very few infiltrating T_R cells and a higher proportion of CD8⁺ T cells.