Mast cells regulate CD4(+) T-cell differentiation in the absence of antigen presentation

Background: Given their unique capacity for antigen uptake, processing, and presentation, antigen-presenting cells (APCs) are critical for initiating and regulating innate and adaptive immune responses. We have previously shown the role of nicotinamide adenine dinucleotide (NAD(+)) in T-cell differentiation independently of the cytokine milieu, whereas the precise mechanisms remained unknown. Objective: The objective of this study is to further dissect the mechanism of actions of NAD(+) and determine the effect of APCs on NAD(+)-mediated T-cell activation. Methods: Isolated dendritic cells and bone marrow-derived mast cells (MCs) were used to characterize the mechanisms of action of NAD(+) on CD4(+) T-cell fate in vitro. Furthermore, NAD(+)-mediated CD4(+) T-cell differentiation was investigated in vivo by using wild-type C57BL/6, MC-/-, MHC class II-/-, Wiskott-Aldrich syndrome protein (WASP)(-/-), 5C.C7 recombination-activating gene 2 (Rag2)(-/-), and CD11b-DTR transgenic mice. Finally, we tested the physiologic effect of NAD(+) on the systemic immune response in the context of Listeria monocytogenes infection. Results: Our in vivo and in vitro findings indicate that after NAD(+) administration, MCs exclusively promote CD4(+) T-cell differentiation, both in the absence of antigen and independently of major APCs. Moreover, we found that MCs mediated CD4(+) T-cell differentiation independently of MHC II and T-cell receptor signaling machinery. More importantly, although treatment with NAD(+) resulted in decreased MHC II expression on CD11c(+) cells, MC-mediated CD4(+) T-cell differentiation rendered mice resistant to administration of lethal doses of L monocytogenes. Conclusions: Collectively, our study unravels a novel cellular and molecular pathway that regulates innate and adaptive immunity through MCs exclusively and underscores the therapeutic potential of NAD(+) in the context of primary immunodeficiencies and antimicrobial resistance.