Investigating murine follicular T cells as well as the contribution of the antagonistic CD155 ligands TIGIT and CD226 to their differentiation and function

• Applicant: Dr. Günter Bernhardt
• Subject Area: Immunology
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271747048

Follicular T cells represent a variety of highly specialized CD4 T cells capable to immigrate into B cell follicles and to govern adaptive immune responses. Of utmost importance for these processes are follicular helper T (TFH) and follicular regulatory T (TFR) cells that counteract TFH driven activities. TFR cells are derived from classical regulatory T cells of thymic origin. In contrast, TFH cells differentiate from naïve CD4 T cells in several steps that finally results in their location in the germinal centre (GC). During a GC reaction, TFH cells assist rapidly dividing B blasts to develop into either plasma or memory cells. They also help GC B cells to perform class switch recombination and hypermutation of the antibodies they express. We could show that the cell surface receptor CD155 and its ligand CD226 are critically involved in the establishment of a regular pool of follicular T cell in Peyers Patches (PP). Moreover, mice lacking CD155 developed only a substandard secondary humoral immune response when antigen was administered orally. Notably, the primary response mediated by IgM was not affected. We established protocols allowing us to more precisely define TFH and TFR cells. As outlined in this proposal, this enabled an improved definition of the phenotypic TFH cell deficiencies caused by lack of CD155 or CD226. Recently, another counter-receptor for CD155 was discovered, named TIGIT. In contrast to CD226 that represents a T cell co-activator, TIGIT was shown to suppress T cell responses. We observed that T cells developing into TFH cells perform a CD155 ligand switch by down-regulating CD226 and up-regulating TIGIT. This suggests that TIGIT may contribute to control TFH cell maintenance and function and that a properly balanced expression of CD226 and TIGIT orchestrates a TFH cells curriculum. With this proposal, we intend to refine our analyses of TFH and TFR cells. To this end, we will perform gene chip analyses. By in vivo migration assays, we will correlate TFH subpopulations defined by their flow cytometric phenotype with their localization inside a lymph node. The new standards will be exploited to investigate development and function of TFH and TFR cells in vivo and in vitro including those lacking TIGIT or CD226. Moreover, T cells genetically manipulated to prevent CD226 down-regulation and/or TIGIT up-regulation will be tested for their capacity to develop into TFH cells in vivo. The proposed studies will focus on cells of PP origin and those coming into existence in periphery following immunization; a comparison of the data will help reveal potential differences in the generation of TFH and TFR cells in these two different immunological compartments. Since available evidence suggests that CD155 is of particular importance in mucosal immunology, we will also study the influence of TIGIT absence on T cells in an asthma model.

• DFG Programme: Research Grants