HVEM-BTLA interaction during human T cell activation

Inhibitory costimulatory signals have important roles in dampening T cell responses, which make them attractive therapeutic targets. Blocking antibodies to the inhibitory receptors PD- 1 and CTLA-4 are already in clinical use to enhance anti-tumor response in patients suffering from melanoma and other cancers. The B and T cell attenuator (BTLA) is a receptor which can potently inhibit T cell responses and consequently blocking BTLA might also promote the eradication of tumors by the immune system. However, BTLA is a peculiar receptor since its ligand is the Herpes virus entry mediator (HVEM), which also has a role as a potent costimulatory receptor. HVEM is part of a complex signaling network and can be triggered by several other ligands. Another unique property of BTLA is that unlike other coinhibitory receptors such as CTLA-4, LAG-3 or PD-1, whose expression is manly limited to activated or exhausted T cells, it can be detected on the majority of T cells including nave CD4 and CD8 T cells. HVEM is extensively co-expressed with BTLA and there is evidence that the co-expression of these molecules regulates their functions. BTLA bears similarities with PD-1, which can be regarded as the primary coinhibitory receptor on T cells but our preliminary results indicate profound differences in the inhibitory signal transduction pathways of receptors. Moreover, BTLA also harbors an activating motif that is implicated in promoting T cell activation and survival. Within the proposed project we will use efficient T cell reporter systems developed in our laboratory to gain mechanistic insight how interaction of HVEM with BTLA and its other binding partners regulate T cell responses. We aim to dissect BTLA signaling pathways by identifying molecules recruited to the cytoplasmic tail of BTLA upon engagement by HVEM. Results obtained in our reporter platform will be confirmed in primary human T cells. In addition, we will perform a series of experiments to identify differences and similarities in the regulation of T cell responses by PD-1 and BTLA. We have observed that HVEM antibodies can have a dual role in promoting HVEM costimulation and preventing BTLA mediated inhibition. We will use PBMCs derived from melanoma patients to test our hypothesis that such antibodies will potently enhance tumor-specific T cell responses. We expect that the results of our study will further our understanding of the unique and complex pathways involving HVEM and BTLA. Moreover, it will help to gauge the therapeutic potential of antibodies targeting HVEM and BTLA.

Activating and inhibitory (also referred to as coinhibitory) costimulatory pathways critically impact on the outcomes of T cell responses. Consequently, they are important targets to enhance the immune responses towards tumors or ameliorate the autoimmune diseases. Our project has focused on a pathway that is very particular since it represents the interaction of an inhibitory receptor (BTLA) with an activating receptor (HVEM). HVEM is the only binding partner for BTLA, whereas HVEM, a member of the Tumor necrosis factor super family (TNFR-SF) is bound by LIGHT and is part of a large network of receptors and ligands. In our study we have highlighted that this interaction generates potent inhibitory signals in cells expressing BTLA but at the same time cells that express HVEM are strongly activated. Another peculiarity of this pathway that both interaction partners can be frequently found on the same cell where they form a complex. We could show that indeed the majority of human T cells co-express HVEM and BTLA. Importantly, the results of our study point to a dominant role of the inhibitory BTLA in this complex since T cell co-expressing both receptors were largely refractory to stimulation via HVEM, whereas inhibition via BTLA was unimpaired. Moreover, we could show that agonistic antibodies to HVEM can enhance immunity in two ways: On the one hand they can block inhibition via BTLA and on the other hand they can stimulate HVEM expressing immune cells. Currently, there are two major strategies to enhance anti-cancer immunity via costimulatory pathways: Triggering costimulatory pathways by agonistic antibodies to activating costimulatory receptors and blocking coinhibitory pathways with antagonistic receptors to coinhibitory receptors. Based on our results we propose that agonistic HVEM antibodies could concomitantly enhance costimulation via HVEM and block coinhibition via BTLA making such antibodies attractive drug candidates in cancer immunotherapy. In collaboration with the group of Prof. Kolch from the University College Dublin we have performed a genome scale investigation on the impact of BTLA signals on gene expression. Genes upregulated during T cell activation were upregulated to a lesser extent. Similarly, we found genes downregulated were less downregulated when BTLA was engaged indicating that BTLA attenuated transcriptomic changes induced by the T cell receptor complex. These results are consistent with the recruitment of inhibitory phosphatases by BTLA and do not point to an activate regulation of gene expression by this receptor. Moreover, we observed that compared to PD1 the capability of BTLA to inhibit T cell responses is considerably higher. Take together, our study provides important insights into the complex role of BTLA in the regulation of T cell response and our findings highlight the potential of this receptor as target in immunotherapeutic approaches.