Abundant stage-dependent Ly49E expression by liver NK cells is not essential for their differentiation and function

Abstract

The NKR Ly49E has several unique characteristics. Unlike most NKRs, Ly49E is highly expressed on fetal NK cells, whereas expression is decreased on bone marrow-derived NK cells in adult mice. To investigate a possible role for Ly49E in NK cell differentiation and function, we have generated an Ly49E KO mouse. Our results show that bone marrow and splenic NK cells are present in normal numbers in Ly49E KO mice, expressing an unaltered panel of NKRs and differentiation markers. Furthermore, cytokine production and cytotoxicity by these cells are unaffected. Surprisingly, WT DX5(-) liver NK cells express high Ly49E levels in fetal and adult mice. Ly49E(+)DX5(-) liver NK cells transferred into Rag-2(-/-)/gc(-/-) mice maintain high Ly49E expression in the liver and differentiate into DX5(+) NK cells in spleen and bone marrow. Ly49E expression is not crucial for liver NK cell differentiation during ontogeny, as the DX5(-)/DX5(+) ratio, the NKR repertoire, and the granzyme B and TRAIL levels are comparable in Ly49E KO versus WT mice, except for lower TRAIL expression on DX5(-) liver NK cells in 20-day-old mice. The TRAIL-, perforin-, and FasL-mediated cytolysis by liver NK cells is unaffected in Ly49E KO mice. Collectively, we show that in addition to high Ly49E expression on fetal NK cells versus low Ly49E expression on conventional NK cells in adult life, Ly49E remains highly expressed on DX5(-) liver NK cells. However, Ly49E expression does not have a crucial role in differentiation and/or function of these NK cells.

The Ly49E Receptor Inhibits the Immune Control of Acute Trypanosoma cruzi Infection

“…Direct inhibition, at this time point, seems less likely since the Ly49E ligand (uPA) has returned to normal levels. Additionally, the fact that the Ly49E receptor is preferentially expressed by NK and NKT cells in younger mice and is also expressed at higher levels on a per cell basis in these mice [(31) and data not shown], also argues for an indirect effect of earlier inhibition of IFN-γ production. Our hypothesis is that the Ly49E receptor is engaged by uPA early during infection, probably as soon as uPA levels are increased, and with a more pronounced effect in young mice harboring a higher proportion of Ly49E + NK cells in the liver than adult mice.…”

“…This likely reflects the variation in Ly49E expression with the mouse age. Indeed, Ly49E is the only Ly49 receptor expressed on neonatal NK cells and is largely absent on conventional peripheral NK cells in adult mice (25, 26), with the exception of CD49b − liver NK cells that express high levels of Ly49E both in newborns and adult mice (31). This latter observation might explain the slight effect that we still observe in 8-week-old mice.…”

“…Heterozygous Ly49E +/− mice (C57BL/6 background) were interbred to obtain homozygous Ly49E +/+ (WT) and Ly49E −/− (KO) mice, which were bred separately thereafter (31). Ly49E WT and KO mice were bred in individually ventilated cages in the SPF animal facility of Ghent University (Ghent, Belgium).…”

“…Also, Ly49E is the only Ly49 receptor expressed on fetal and neonatal NK cells (25, 26), while in adult tissues, Ly49E expression is largely restricted to NKT cells, skin Vγ3 T cells, and intestinal intraepithelial T lymphocytes (27–29). Conventional peripheral NK cells in adult mice have low Ly49E expression levels (25, 28, 30); however, tissue-resident CD49a + CD49b − liver NK cells express high Ly49E levels, and this both in fetal, newborn, and adult mice (31). …”

“…This, combined with the high expression of the inhibitory Ly49E receptor on liver NK and NKT cells (25, 31), with the induction of macrophages to secrete high levels of plasminogen activator upon T. cruzi infection (9), and with the triggering of the Ly49E receptor by uPA (24), led us to hypothesize that Ly49E plays a regulatory role in T. cruzi infection. We show that plasma uPA levels are increased during T. cruzi infection.…”