In an adaptive immune response, naïve T cells proliferate during infection and generate long-lived memory cells that undergo secondary expansion following re-encounter with the same pathogen. Although Natural Killer cells traditionally have been classified as cells of the innate immune system, they share many similarities with cytotoxic T lymphocytes. In a mouse model of cytomegalovirus (MCMV) infection, we demonstrate that, like T cells, NK cells bearing the virus-specific Ly49H receptor proliferate 100-fold in the spleen and 1000-fold in the liver following infection. Following a contraction phase, Ly49H+ NK cells reside in lymphoid and non-lymphoid organs for several months. These self-renewing “memory” NK cells rapidly degranulate and produce cytokines upon reactivation. Adoptive transfer of these NK cells into naïve animals followed by viral challenge results in a robust secondary expansion and protective immunity. These findings reveal novel properties of NK cells previously attributed only to cells of the adaptive immune system.

During a screen for ethylnitrosourea-induced mutations in mice affecting blood natural killer (NK) cells, we identified a strain, designated Duane, in which NK cells were reduced in blood and spleen but increased in lymph nodes (LNs) and bone marrow (BM). The accumulation of NK cells in LNs reflected a decreased ability to exit into lymph. This strain carries a point mutation within Tbx21 (T-bet), which generates a defective protein. Duane NK cells have a 30-fold deficiency in sphingosine-1-phosphate receptor 5 (S1P5) transcript levels, and S1P5-deficient mice exhibit an egress defect similar to Duane. Chromatin immunoprecipitation confirms binding of T-bet to the S1pr5 locus. S1P-deficient mice exhibit a more severe NK cell egress block, and the FTY720-sensitive S1P1 also plays a role in NK cell egress from LNs. S1P5 is not inhibited by CD69, a property that may facilitate trafficking of activated NK cells to effector sites. Finally, the accumulation of NK cells within BM of S1P-deficient mice was associated with reduced numbers in BM sinusoids, suggesting a role for S1P in BM egress. In summary, these findings identify S1P5 as a T-bet–induced gene that is required for NK cell egress from LNs and BM.

Responsiveness to interleukin-12, but not interferon-γ, is essential for the generation of long-lived natural killer cells capable of responding to secondary viral infection.

It has been reported that an insulin 2 gene knockout, when bred onto nonobese diabetic (NOD) mice, accelerates diabetes. We produced insulin 1 gene knockout congenic NOD mice. In contrast to insulin 2, diabetes and insulitis were markedly reduced in insulin 1 knockout mice, with decreased and delayed diabetes in heterozygous females and no insulitis and diabetes in most homozygous female mice. Lack of insulitis was found for insulin 1 female homozygous knockout mice at 8, 12, and 37 weeks of age. Despite a lack of insulitis, insulin 1 homozygous knockout mice spontaneously expressed insulin autoantibodies. Administration of insulin peptide B:9 -23 of both insulin 1 and 2 to NOD mice induced insulin autoantibodies. Insulin 1 is not the only lymphocytic target of NOD mice. Insulin 1 homozygous knockout islets, when transplanted into recently diabetic wild-type NOD mice, became infiltrated with lymphocytes and only transiently reversed diabetes. These observations indicate that loss of either insulin gene can influence progression to diabetes of NOD mice and suggest that the preproinsulin 1 gene is crucial for the spontaneous development of NOD insulitis and diabetes.

Although major histocompatibility complex (MHC) class II-restricted CD4 T cells are well appreciated for their contribution to peripheral tolerance to tissue allografts, little is known regarding MHC class I-dependent reactivity in this process. Here we show a crucial role for host MHC class I-dependent NK cell reactivity for allograft tolerance in mice induced through either costimulation blockade using CD154-specific antibody therapy or by targeting LFA-1 (also known as CD11a). Tolerance induction absolutely required host expression of MHC class I, but was independent of CD8 T cell-dependent immunity. Rather, tolerance required innate immunity involving NK1.1(+) cells, but was independent of CD1d-restricted NKT cells. Therefore, NK cells seem to be generally required for induction of tolerance to islet allografts. Additional studies indicate that CD154-specific antibody-induced allograft tolerance is perforin dependent. Notably, NK cells that are perforin competent are sufficient to restore allograft tolerance in perforin-deficient recipients. Together, these results show an obligatory role for NK cells, through perforin, for induction of tolerance to islet allografts.

Madera et al. show that NK cells unable to receive type I IFN signals during MCMV infection have defective expansion and memory cell formation, possessing increased susceptibility to apoptosis due to NK cell–mediated fratricide.

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Rationale While microvascular injury is associated with chronic rejection, the cause of tissue ischemia during alloimmune injury is not yet elucidated. Objective We investigated the contribution of T lymphocytes and complement to microvascular injury-associated ischemia during acute rejection of mouse tracheal transplants. Methods and Results Using novel techniques to assess microvascular integrity and function, we evaluated how lymphocyte subsets and complement specifically affect microvascular perfusion and tissue oxygenation in MHC-mismatched transplants. To characterize T cell effects on microvessel loss and recovery, we transplanted functional airway grafts in the presence and absence of CD4+ and CD8+ T cells. To establish the contribution of complement-mediated injury to the allograft microcirculation, we transplanted C3-deficient and C3-inhibited recipients. We demonstrated that CD4+ T cells and complement are independently sufficient to cause graft ischemia. CD8+ T cells were required for airway neovascularization to occur following CD4-mediated rejection. Activation of antibody-dependent complement pathways mediated tissue ischemia even in the absence of cellular rejection. Complement inhibition by CR2-Crry attenuated graft hypoxia, complement/antibody deposition on vascular endothelium and promoted vascular perfusion by enhanced angiogenesis. Finally, there was a clear relationship between the burden of tissue hypoxia (ischemia × time duration) and the development of subsequent airway remodeling. Conclusions These studies demonstrated that CD4+ T cells and complement operate independently to cause transplant ischemia during acute rejection and that sustained ischemia is a precursor to chronic rejection.