Specialized Roles of Human Natural Killer Cell Subsets in Kidney Transplant Rejection

Abstract

Background: Human natural killer (NK) cells are key functional players in kidney transplant rejection. However, the respective contributions of the two functionally distinct human NK cell subsets (CD56 bright cytokine-producing vs. CD56 dim cytotoxic effector) in episodes of allograft rejection remain uncertain, with current immunohistochemical methods unable to differentiate these discrete populations. We report the outcomes of an innovative multi-color flow cytometric-based approach to unequivocally define and evaluate NK cell subsets in human kidney allograft rejection. Methods: We extracted renal lymphocytes from human kidney transplant biopsies. NK cell subsets were identified, enumerated, and phenotyped by multi-color flow cytometry. Dissociation supernatants were harvested and levels of soluble proteins were determined using a multiplex bead-based assay. Results were correlated with the histopathological patterns in biopsies—no rejection, borderline cellular rejection, T cell-mediated rejection (TCMR), and antibody-mediated rejection (AMR). Results: Absolute numbers of only CD56 bright NK cells were significantly elevated in TCMR biopsies. In contrast, both CD56 bright and CD56 dim NK cell numbers were significantly increased in biopsies with histopathological evidence of AMR. Notably, expression of the activation marker CD69 was only significantly elevated on CD56 dim NK cells in AMR biopsies compared with no rejection biopsies, indicative of a pathogenic phenotype for this cytotoxic NK cell subset. In line with this, we detected significantly elevated levels of cytotoxic effector molecules (perforin, granzyme A, and granulysin) in the dissociation supernatants of biopsies with a histopathological pattern of AMR. Conclusions: Our results indicate that human NK cell subsets are differentially recruited and activated during distinct types of rejection, suggestive of specialized functional roles.

Perirenal Adipose Tissue Displays an Age-Dependent Inflammatory Signature Associated With Early Graft Dysfunction of Marginal Kidney Transplants

“…In kidney transplant recipients, innate NK cells have recently been identified as a key effector mechanism regulating the level of endothelial lesion and repair as well as vascular rejection and allograft vasculopathy (11,25,(42)(43)(44). NK cells have also been reported to contribute to immune senescence in kidney transplant candidates (45).…”

The Role of Natural Killer Cells in the Immune Response in Kidney Transplantation

“…Biopsies from patients with T cell-mediated rejection showed an increased absolute number of CD56 bright NK cells while in the biopsies of patients with antibody-mediated rejection both CD56 bright and CD56 dim NK cells were increased. Only CD56 dim showed the expression of activation markers such as CD69 and high levels of cytotoxic effector molecules (perforin, granzyme A, and granulysin) in supernatants obtained from ABMR biopsies (60). Once again, these data highlight the importance of CD56 dim cells activation by the micro-environment featuring ABMR, where they can guide vascular damage.…”

“…These findings suggest that NK cells need to be carefully evaluated, because variations in NK cell marker expression might be associated with the activation of different immune pathways in graft rejection. A recently published study, where authors isolated lymphomonocytes directly from graft biopsies and used a multi-color flow cytometry to define NK cell subsets involved in different graft rejection, confirmed this hypothesis (60). Biopsies from patients with T cell-mediated rejection showed an increased absolute number of CD56 bright NK cells while in the biopsies of patients with antibody-mediated rejection both CD56 bright and CD56 dim NK cells were increased.…”

“…Once again, these data highlight the importance of CD56 dim cells activation by the micro-environment featuring ABMR, where they can guide vascular damage. CD56 bright NK cells can instead play a specific role in TCMR through the secretion of pro-inflammatory molecules such as IFN-g ( Figure 1A), which increase the recruitment of alloreactive T cells and up-regulate HLA alloantigens (MHC I and II) on graft target cells, making them more susceptible to cytotoxic killing (60).…”

HLA Class I Molecules as Immune Checkpoints for NK Cell Alloreactivity and Anti-Viral Immunity in Kidney Transplantation

“…Infiltration of CD56 pos NK cells has been associated with poor outcome of kidney transplantation and interstitial fibrosis ( 33 , 34 ). A more recent study, used flow cytometry to obtain more in depth profiles of NK cells revealing that CD56 bright NK cell infiltrates were enhanced in biopsies from patients experiencing T cell mediated rejection (TCMR) while CD56 dim NK cell infiltrates characterized biopsies from patients with antibody mediated rejection (AMR) ( 35 ). Since CD56 bright NK cells are potent producers of cytokines, this was suggestive of a role for CD56 bright NK cells in the recruitment and activation of alloreactive T cells ( 35 ).…”