Signal Regulatory Protein α (SIRPα)+ Cells in the Adaptive Response to ESAT-6/CFP-10 Protein of Tuberculous Mycobacteria

Background: Early secretory antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) are co-secreted proteins of Mycobacterium tuberculosis complex mycobacteria ( includes M. bovis, the zoonotic agent of bovine tuberculosis) involved in phagolysosome escape of the bacillus and, potentially, in the efficient induction of granulomas. Upon tuberculosis infection, multi-nucleate giant cells are elicited, likely as a response aimed at containing mycobacteria. In tissue culture models, signal regulatory protein (SIRP)alpha ( also referred to as macrophage fusion receptor or CD172a) is essential for multi-nucleate giant cell formation. Methodology/Principal Findings: In the present study, ESAT-6/CFP-10 complex and SIRP alpha interactions were evaluated with samples obtained from calves experimentally infected with M. bovis. Peripheral blood CD172a(+) (SIRP alpha-expressing) cells from M. bovis-infected calves proliferated upon in vitro stimulation with ESAT-6/CFP-10 ( either as a fusion protein or a peptide cocktail), but not with cells from animals receiving M. bovis strains lacking ESAT-6/CFP-10 (i.e, M. bovis BCG or M. bovis Delta RD1). Sorted CD172a(+) cells from these cultures had a dendritic cell/macrophage morphology, bound fluorescently-tagged rESAT-6:CFP-10, bound and phagocytosed live M. bovis BCG, and co-expressed CD11c, DEC-205, CD44, MHC II, CD80/86 (a subset also co-expressed CD11b or CD8 alpha). Intradermal administration of rESAT-6: CFP-10 into tuberculous calves elicited a delayed type hypersensitive response consisting of CD11c(+), CD172a(+), and CD3(+) cells, including CD172a-expressing multinucleated giant cells. Conclusions/Significance: These findings demonstrate the ability of ESAT-6/CFP-10 to specifically expand CD172a(+) cells, bind to CD172a(+) cells, and induce multi-nucleated giant cells expressing CD172a.