Natural killer cells have been recognized as early antitumor effectors through their direct cytotoxic functions.(4) Further recent evidence strongly supports that NK cells are important regulatory cells in antitumor immune responses by providing regulatory cytokines such as IFN-γ.(6,11) Herein, we characterized the distribution of tumor-infiltrating NK cell subpopulations, their activation status and their IFN-γ production within tumors. Mature Mac-1high CD27high NK cells infiltrated early MCA205 tumors and they displayed an activated cell surface phenotype and provided early IFN-γ. Importantly, we also found that host IFN-γ was critical for NK cell infiltration into tumors, implicating the importance of IFN-γ as a positive regulatory factor for NK cell recruitment to tumors.
By investigating the expression of CD27 on naïve NK cells together with other NK cell maturation and differentiation markers, we previously showed that CD27 expression dissects the mature NK cell population into CD27high and CD27lo subsets.(16,22) Both types of mature Mac-1high NK cells exert typical NK cell functions; however, CD27high cells have a lower threshold for activation than CD27lo cells whose activation is tightly regulated.(16,22) In addition to the effector function of NK cells, the diversity in tissue distribution of NK cells might be important for tissue resident/specific immune responses. In mice, the tissue distribution of NK cell subsets in various lymphoid and non-lymphoid organs is quite distinct.(16,22) Interestingly, the CD27lo subset is comparatively excluded from bone marrow and lymph node, and is the dominant tissue-resident NK cell population in peripheral tissues such as lung or blood.(16,22) In addition to such steady-state tissue-resident cells, NK cells appear to be rapidly recruited to tissues at the site of immune responses, and chemokines might play an important role in this process. Regarding the chemokine receptor expression on NK cells, it has been clearly shown that the chemokine receptor CXCR3 plays an important role in NK cell recruitment and the expression of CXCR3 ligands generally can be upregulated by IFN-γ.(12,16,28) Among NK cell subpopulations, Mac-1lo CD27high and Mac-1high CD27high NK cells constitutively express CXCR3, whereas there was no detectable CXCR3 expression on CD27lo NK cells.(16,22) In the present study, we observed that the expression of CXCR3 on tumor-infiltrating NK cells was severely downregulated, whereas the expression of CD69 was highly upregulated. Because such CXCR3 downregulation was not observed in splenic NK cells in the same tumor-bearing mice, we assume the tumor microenvironment might preferentially down modulate NK cell CXCR3 expression. Considering the general importance of CXCR3 in NK cell mobilization,(12,16,28) the downregulation of CXCR3 on tumor-infiltrating NK cells might be a key event that limits NK cell egress from the tumor microenvironment, maintaining NK cell immune surveillance. Alternatively, pre-activated NK cells might be preferentially recruited into the tumor since in vivo activation of NK cells leads to a similar subset distribution and CXCR3 downregulation of NK cells.(29) Regarding the upregulation of CD69 expression on tumor-infiltrating NK cells, there are several studies reporting that CD69 molecules negatively regulate NK cell antitumor immune responses.(30,31) Further study will be important in the context of NK cell homeostasis within the tumor microenvironment to further understand the detailed mechanisms of tumor escape from NK cells.
Such a critical requirement of host IFN-γ was also seen in NK cell mobilization to other organs including implanted tumors with extracellular matrix Matrigel;(27,28,32) collectively the data indicate the general importance of IFN-γ in maintaining NK cell infiltration to inflamed tissues. Of note, we observed that CD8 T-cell infiltration into MCA205 tumor was also impaired in IFN-γ−/− mice (data not shown), thus suggesting the importance of IFN-γ as a key cytokine for maintaining antitumor effector cells within the tumor microenvironment. Despite the clear requirement of IFN-γ in NK cell tumor infiltration, the detailed mechanism by which it regulates NK cell accumulation into the tumor microenvironment has not yet been determined. Further study will be required to fully understand NK cell behavior in the tumor microenvironment and successful establishment of effective immunotherapy for cancer.