Innate immune cells remember: Immunological memory is a phenomenon by which immune cells can quickly recognize an antigen that the host has previously encountered. Certain cells of the innate immune system exhibit memory-like responses know as trained immunity. Rapid, antigen-specific secondary (anamnestic) responses were long thought to be the domain of B and T cells. However, Dai et al. report that monocytes and macrophages can acquire memory specific for particular major histocompatibility complex I antigens using paired A-type immunoglobulin-like receptors (PIR-As) (see the Perspective by Dominguez-Andrés and Netea). This pathway contributes to recognition and rejection of allograft-transplanted tissue from a donor of the same species. Genetic depletion or blockade of PIR-As in mice diminished the rejection of kidney and heart allografts. This work, which expands immunological memory to include myeloid cells, points to targets that may improve organ transplantation outcomes in the future. Science , this issue p. 1122 ; see also p. 1052

Tissue-resident memory T cells (T_RM) contained at sites of previous infection provide local protection against reinfection. Whether they form and function in organ transplants where cognate antigen persists is unclear. This is a key question in transplantation as T cells are detected long term in allografts, but it is not known whether they are exhausted or are functional memory T cells. Using a mouse model of kidney transplantation, we showed that antigen-specific and polyclonal effector T cells differentiated in the graft into T_RM and subsequently caused allograft rejection. T_RM identity was established by surface phenotype, transcriptional profile, and inability to recirculate in parabiosis and retransplantation experiments. Graft T_RM proliferated locally, produced interferon-γ upon restimulation, and their in vivo depletion attenuated rejection. The vast majority of antigen-specific and polyclonal T_RM lacked phenotypic and transcriptional exhaustion markers. Single-cell analysis of graft T cells early and late after transplantation identified a transcriptional program associated with transition to the tissue-resident state that could serve as a platform for the discovery of therapeutic targets. Thus, recipient effector T cells differentiate into functional graft T_RM that maintain rejection locally. Targeting these T_RM could improve renal transplant outcomes.

Our understanding of tissue-resident memory T (T _RM ) cell biology has been largely developed from acute infection models in which antigen is cleared and sterilizing immunity is achieved. Less is known about T _RM cells in the context of chronic antigen persistence and inflammation. We investigated factors that underlie T _RM maintenance in a kidney transplantation model in which T _RM cells drive rejection. In contrast to acute infection, we found that T _RM cells declined markedly in the absence of cognate antigen, antigen presentation, or antigen sensing by the T cells. Depletion of graft-infiltrating dendritic cells or interruption of antigen presentation after T _RM cells were established was sufficient to disrupt T _RM maintenance and reduce allograft pathology. Likewise, removal of IL-15 transpresentation or of the IL-15 receptor on T cells during T _RM maintenance led to a decline in T _RM cells, and IL-15 receptor blockade prevented chronic rejection. Therefore, antigen and IL-15 presented by dendritic cells play nonredundant key roles in CD8 T _RM cell maintenance in settings of antigen persistence and inflammation. These findings provide insights that could lead to improved treatment of chronic transplant rejection and autoimmunity.

Donor-recipient mismatches in histocompatibility antigens recognized by lymphoid cells have been demonstrated to adversely affect allograft outcomes. In contrast, it remains unclear whether mismatches sensed by innate myeloid cells have a similar effect. We investigated the consequences of mismatch in the polymorphic gene encoding signal regulatory protein α (SIRPα) on kidney allograft pathology and survival in mice and humans. We found that SIRPα variants elicit monocyte activation by binding to CD47 and that eliminating SIRPα mismatch or recipient CD47 expression prevented chronic allograft pathology in mice receiving major histocompatibility complex (MHC)–mismatched renal allografts. Human genomic analysis identified two haplotype categories, A and B, encoding SIRPα variants with distinct CD47 binding interfaces. In kidney transplant recipients ( N = 455), SIRPα mismatch was associated with increased acute rejection and graft fibrosis in the first posttransplant year, and A recipients of B kidneys had reduced long-term graft survival (hazard ratio, 3.2; 95% confidence interval, 1.5 to 6.9; P = 0.002), a finding that was confirmed in an independent validation cohort ( N = 258). Moreover, monocytes in these graft recipients had an activated phenotype. The effects of SIRPα mismatch were independent of ancestry, human leukocyte antigen mismatch, donor-specific antibodies, and delayed graft function. Therefore, these data demonstrate that a donor-recipient mismatch that causes innate immune activation is a determinant of kidney transplantation outcomes.

Background. Renal ischemia‐reperfusion injury (RIRI) plays an important role in the poor prognosis of patients with renal transplants. However, the potential targets and mechanism of IRI are still unclear. Method. Differential gene expression (DEG) analysis and weighted correlation network analysis (WGCNA) were performed on the GSE27274 dataset. Pathway enrichment analysis on the DEGs was performed. To identify the hub DEGs, we constructed a protein‐protein interaction (PPI) network. Finally, the hub genes were verified, and candidate drugs were screened from the DsigDB database. Results. A hundred DEGs and four hub genes (Atf3, Psmb6, Psmb8, and Psmb10) were screened out. Pathway enrichment analysis revealed that 100 DEGs were mainly enriched in apoptosis and the TNF signaling pathway. The four hub genes were verified in animal models and another dataset (GSE148420). Thereafter, a PPI network was used to identify the four hub genes (Atf3, Psmb6, Psmb8, and Psmb10). Finally, eight candidate drugs were identified as potential drugs. Conclusion. Three hub genes (Psmb6, Psmb8, and Psmb10) were associated with RIRI and could be potential novel biomarkers for RIRI.