Effect of Borrelia burgdorferi-SIRPalpha Interaction on the Immune Response - Innate immunity is the first line of defense against pathogens, and it is critically reliant on the efficient elimination of the invading agents by phagocytosis. Phagosomal digestion enables microbial killing, antigen presentation to activate adaptive immunity and establish immune memory, and the activation of appropriate inflammatory responses. It has recently become clear that the tick-borne pathogen B. burgdorferi (Bb), an agent of Lyme disease, can evade phagocytosis by macrophages through a novel mechanism. Molecular mimicry of the “don’t eat me” signals that block phagocytosis of host cells allows Bb to engage SIRPα (signal regulatory protein alpha) on the surface of macrophages. CD47 is known to be upregulated by cancer cells and virally infected cells, thus allowing them to escape phagocytic clearance. More recently, it’s become clear that this innate immune checkpoint is hijacked by the Bb virulence protein, P66, that functionally mimics CD47. P66 is encoded by all Borrelia species, and binds to the phagocyte receptor SIRPα with high affinity, reducing Bb phagocytosis. Evasion of phagocytic clearance is likely an essential component of Bb’s ability to cause persistent infection, and likely to be critical for both maintenance of the enzootic cycle and the debilitating disease observed in many patients. Genetic deletion of p66 increases macrophage phagocytosis of Bb in addition to blocking P66’s well-established porin and integrin-binding functions. Thus, Bb mutants that do not produce P66 (p66::kana; termed Dp66 here) are not infectious in mice, as they are detectable by quantitative PCR (qPCR) or culture only at the site of inoculation ~ 1-2 days post-injection, while complementation of p66 to its endogenous genetic locus (p66cc) reconstitutes disseminated infection longer than 2 days in duration (hereafter referred to as “infectivity”) in mice. Phagocytosis is important for pathogen clearance, antigen presentation, adaptive immunity, and appropriate inflammation, thus we will determine how engagement of SIRPa by Bb P66 subverts the immune response. Our hypothesis is that P66-SIRPa interaction diminishes the development of effective innate and adaptive immune responses to Bb in mice. We will test this hypothesis using complementary approaches targeting both bacterial P66 and host SIRPa, leveraging both microbiological and immunologic approaches to illuminate this novel mechanism of immune evasion. Using commercially available mice, we will quantify Bb loads and dissemination kinetics, and murine antibody and cellular responses to the bacteria over six infection times. Since bacterial targeting of SIRPa is, to date, novel, we expect this work to shed light on the fundamental biology of Bb and its host-dependent life cycle, particularly the novel role of a bacterial “don’t eat me” surface protein and how it affects the immune response that does not clear the infection in immunocompetent hosts.
