TACI deficiency leads to alternatively activated macrophage phenotype and susceptibility to Leishmania infection

The TNF family member, transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), is a key molecule for plasma cell maintenance and is required in infections where protection depends on antibody response. Here, we report that compared with WT mouse, TACI KO M phi s expressed lower levels of Toll-like receptors (TLRs), CD14, myeloid differentiation primary response protein 88, and adaptor protein Toll/IL-1 receptor domaincontaining adapter-inducing IFN-beta and responded poorly to TLR agonists. Analysis of M phi phenotype revealed that, in the absence of TACI, M phi s adapt the alternatively activated (M2) phenotype. Steady-state expression levels for M2 markers IL-4R alpha, CD206, CCL22, IL-10, Arg1, IL1RN, and FIZZ1 were significantly higher in TACI KO M phi than in WT cells. Confirming their M2 phenotype, TACI-KO M phi s were unable to control Leishmania major infection in vitro, and intradermal inoculation of Leishmania resulted in a more severe manifestation of disease than in the resistant C57BL/6 strain. Transfer of WT M phi s to TACI KO mice was sufficient to significantly reduce disease severity. TACI is likely to influence M phi phenotype by mediating B cell-activating factor belonging to the TNF family (BAFF) and a proliferation inducing ligand (APRIL) signals because both these ligands down-regulated M2 markers in WT but not in TACI-deficient M phi s. Moreover, treatment of M phi s with BAFF or APRIL enhanced the clearance of Leishmania from cells only when TACI is expressed. These findings may have implications for understanding the shortcomings of host response in newborns where TACI expression is reduced and in combined variable immunodeficiency patients where TACI signaling is ablated.