C5aR contributes to the weak Th1 profile induced by an outbreak strain of Mycobacterium tuberculosis.

C5a anaphylatoxin is a component of the complement system involved in the modulation of T-cell polarization. Herein we investigated whether C5a receptors, C5aR and C5L2, modulate the cytokine profiles induced by Mycobacterium tuberculosis (Mtb). We analyzed the impact of both receptors on T helper cell polarization induced by the multidrug resistant outbreak strain named M, which is a poor IFN-γ inducer compared with the laboratory strain H37Rv. To this aim, we first blocked C5aR or C5L2 of peripheral blood monocytes (Mo) from patients with tuberculosis and healthy donors, then we stimulated the Mo either with H37Rv or the M strain, and finally we analyzed cytokine profiles of Mo/macrophages (MΦ) and CD4+ T-cells. We found that: (i) Mtb modulated the expression of both C5a receptors, (ii) C5aR inhibited the expansion of CD4+IFN-γ+ lymphocytes stimulated by the M strain but not by H37Rv, (iii) both receptors modulated the Mo/MΦ cytokine expression induced by Mtb. We conclude that C5aR, but not C5L2, plays a role in T helper cell polarization induced by Mtb and that this effect is strain- and donor-dependent. We speculate that the epidemiologically successful M strain takes advantage of this C5aR-mediated inhibition of Th1 polarization to survive within the host.