Characterization of GM- and M-CSF-Derived Macrophages Response to Lymphocytic Choriomeningitis Virus Infection and TLR7 Agonist
Macrophages (MΦ) are innate immune cells considered as the primary line of defense during infection and as controllers of tissue homeostasis. In response to viral infection, MΦ induce cytokine production and regulate adaptive immunity via antigen presentation to T cells. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) are two key cytokines needed for the differentiation of MФ. M-CSF MФ have been used as models to study virus infection; however, the responses to viral infection by GM-CSF MФ have not been well characterized. We evaluated the production of cytokines by MФ in response to two strains of lymphocytic choriomeningitis virus (LCMV), LCMV Armstrong (LCMV-ARM) and LCMV clone 13 (LCMV-Cl13). We found that LCMV-Cl13 and -ARM infection induced the production of different patterns of pro-inflammatory cytokines, including IL-6 and TNF-α, but failed to produce IL-12p70 and IL-23 by GM-CSF MФ. Conversely, M-CSF MФ induced more anti-inflammatory cytokine IL-10. Moreover, these MФ responded differently to the viral strains; with LCMV-ARM, GM-CSF MФ and M-CSF MФ induced high IL-6 and IL-10, respectively, whereas LCMV-Cl13 GM-CSF MФ generated more TNF-α. Since TLR7 is one of the main innate immune receptors that recognise ssRNA viruses, we examined MФ responsiveness to TLR7-mediated signaling pathways after LCMV-ARM infection following R848 (resiquimod) treatment. We found that R848-mediated signaling molecule activation was reduced in both MФ subsets during the early infection. However, GM-CSF MФ induced higher pro-inflammatory cytokine expression (e.g. IL-6, TNF-a, and IL-12p40) and M-CSF MФ exhibited higher anti-inflammatory cytokine expression (eg IL-10). Moreover, TLR7 expression was maintained in M-CSF MФ in response to LCMV infection and increased in GM-CSF MФ, but TLR7 expression was partially decreased in response to R848 treatment in both M-CSF and GM-CSF MФ. Finally, we tested antagonism between GM-CSF and M-CSF in cultures of MФ with varying ratios of M-CSF:GM-CSF. We observed that GM-CSF can be dominant over M-CSF in terms of MФ polarization, cytokine production, and MФ morphology. We showed that only minimal GM-CSF in the mixed culture ratios resulted in MФ polarization towards an M1 phenotype, with regards to NO production, cytokines induction, and MФ morphology.