In innate immune cells, the expression of pattern recognition receptors (PRRs), such as Toll-like, RIG-I-like, or NOD-like receptors (TLRs, RLRs, or NLRs) is crucial for the early detection of infection. Engagement of these receptors is essential for the onset of the inflammatory response and for the efficient presentation of foreign antigens to T lymphocytes.
In our group, we are interested in the study of PRRs, particularly in T cells. In fact, the role of these receptors has been well characterized in innate immune cells. However, specific PRRs are highly expressed by T lymphocytes, an aspect of adaptive immune responses that has been overlooked thus far. It is possible that these PRRs may provide T lymphocytes with functions resembling that of innate immune cells. Alternatively, proteins belonging to the PRR families may exert novel functions in roles more traditionally associated with T cell biology (adaptive immune system pathways).
One focus of our research is the study of novel NLRs poorly characterized and highly expressed in T lymphocytes. By using Nlrc5-deficient mice, we recently showed that NLRC5 is not involved in innate sensing, but acts instead as a transcriptional regulator of major-histocompatibility complex (MHC) class I, particularly in lymphocytes (Figure 1).
Fig. 1. Histograms show the expression of MHC class I (H-2K) on Nlrc5-deficient (Nlrc5 ko) and control (Nlrc5 floxed) T cells as measured by flow cytometry.
In the future, we would like to better understand different molecular aspects of NLRC5 activity, as schematically represented in Figure 2.
Fig. 2. Different molecular aspects of NLRC5 activity we would like to better understand: 1) which factors are involved in the transcriptional regulatory activity of NLRC5? 2) Which additional genes are controlled by NLRC5? 3) Are specific factors recruiting NLRC5 to the promoter region of MHC class I genes?
Moreover, the downregulation of MHC molecules is an established mechanism exploited by transformed or infected cells to evade immunosurveillance. We thus propose to explore the role of NLRC5 in preventing these pathologies by taking advantage of novel conditional Nlrc5-deficient mice and assessing NLRC5 expression and activity in primary human specimens. Indeed, we already found low NLRC5 expression in a panel of human and mouse lymphoid tumor cell lines (Figure 3).
Fig. 3. NLRC5 expression in different human lymphoid tumor cell lines was assessed by immunoblot analysis (HD: healthy donor controls).
As anticipated by our data on NLRC5, we expect that a detailed analysis of PRR function in T cells will reveal unexpected adaptive aspects of PRR signaling, or highlight unappreciated innate features of T lymphocytes. Both scenarios would be equally innovative and relevant to our knowledge of vertebrate immunology. This may lead us to reconsider some of the features distinguishing innate from adaptive immunity, and may provide us with new opportunities for therapeutic intervention.