NK cells modulate T cell responses via interaction with dendritic cells in Chlamydophila pneumoniae infection

Protective immune response to chlamydial infection is largely dependent on cell-mediated immune responses with IFN-γ production. Recent studies have shown the critical role of NK cells in bridging innate and adaptive immune responses. In this study, we investigated the effect of NK cells on T cell responses during Chlamydophila pneumoniae (Cpn) lung infection. The results showed that NK cells play a protective role in Cpn infection and influence T cell immunity largely though modulating dendritic cells (DCs) function. Specifically, we found that NK depletion significantly impaired type 1 T cell responses, but enhanced FOXP3⁺Treg cells and IL-10-producing CD4⁺T cells. The alteration of T cell responses was associated with more disease severity and higher chlamydial growth in the lung. Further analysis of DC phenotype and cytokine profile found that DCs from NK cell-depleted mice expressed lower levels of co-stimulatory molecules and produced higher levels of IL-10 than those from control IgG-treated mice. More importantly, the adoptive transfer of DCs from NK cell-depleted mice induced a much lower degree of type 1 T cell responses but a higher amount of FOXP3⁺ Treg cells and IL-10-producing CD4⁺T cells in the recipient mice than DCs from IgG-treated mice. In contrast to the strong protective effect observed in recipients of DCs from IgG-treated mice, the recipients of DCs from NK cell-depleted mice failed to be protected against Cpn infection. The data suggest that NK cells play a critical role in coordinating innate and adaptive immunity in Cpn lung infection by modulating the DC function to influence T cell responses.