Natural Killer Cell-specific Genes: Klf12 and Spry2
Natural killer (NK) cells are innate lymphocytes that play in integral role in tumor rejection and viral clearance. Unlike their lymphocyte counterparts, NK cells have the unique ability to recognize and lyse target cells without prior exposure. However, there is no NK cell-specific protein that is exclusively expressed by NK cells. Therefore, identification of a NK cell-specific protein will allow better understanding of why NK cells are unique cytotoxic lymphocytes. From the Immunological Genome Consortium (ImmGen) studies, we identified two proteins preferentially expressed in mouse NK cells: 1) KLF12, a novel transcription factor, and 2) SPRY2, a negative regulator of receptor tyrosine kinase signaling. I examined mouse NK cell development and effector functions to understand the role of KLF12 and SPRY2 in NK cells. I generated Klf12 conditional knockout mice and found that KLF12 is dispensable for NK cell development, IFN-γ production, degranulation, and NK cell proliferation in intact non-competitive mice. The lack of a phenotype was perhaps due to redundancy or compensatory mechanisms. Therefore, I performed RNA-sequencing analysis with ImmGen and observed increased expression of Btg3, an anti-proliferative gene, in KLF12-deficient NK cells compared to wildtype (WT) NK cells. Interestingly, competitive mixed bone marrow (BM) chimeric mice exhibited reduced KLF12-deficient NK cell development, altered IFN-γ production and degranulation, and severe impairment of NK cell proliferation in vitro and in vivo in response to mouse cytomegalovirus (MCMV) infection. KLF12-deficient NK cells also expressed higher levels of the interleukin-21 receptor (IL-21R), which resulted in increased IL-21R signaling and greater inhibition of NK cell proliferation. Thus, KLF12 regulates NK cell proliferation. By analyzing mice with NK cell-specific deletion of Spry2, I found that SPRY2 is dispensable for NK cell development, IFN-γ production, and degranulation. SPRY2-deficient NK cells expressed more inhibitory NK cell receptors than WT NK cells. Furthermore, SPRY2-deficient NK cells were not maintained when cultured in media alone without survivial cytokines even though their proliferative capacity was unperturbed. In an adoptive transfer model of MCMV infection, SPRY2-deficient NK cell expansion was impaired compared to WT NK cells, further suggesting that SPRY2 may regulate NK cell survival and/or apoptosis.