Effector Memory-Expressing CD45RA (TEMRA) CD8+ T Cells from Kidney Transplant Recipients Exhibit Enhanced Purinergic P2X4 Receptor-Dependent Proinflammatory and Migratory Responses.

BACKGROUND: The mechanisms regulating CD8+ T cell migration to nonlymphoid tissue during inflammation have not been fully elucidated, and the migratory properties of effector memory CD8+ T cells that re-express CD45RA (TEMRA CD8+ T cells) remain unclear, despite their roles in autoimmune diseases and allotransplant rejection. METHODS: We used single-cell proteomic profiling and functional testing of CD8+ T cell subsets to characterize their effector functions and migratory properties in healthy volunteers and kidney transplant recipients with stable or humoral rejection. RESULTS: We showed that humoral rejection of a kidney allograft is associated with an accumulation of cytolytic TEMRA CD8+ T cells in blood and kidney graft biopsies. TEMRA CD8+ T cells from kidney transplant recipients exhibited enhanced migratory properties compared with effector memory (EM) CD8+ T cells, with enhanced adhesion to activated endothelium and transmigration in response to the chemokine CXCL12. CXCL12 directly triggers a purinergic P2×4 receptor-dependent proinflammatory response of TEMRA CD8+ T cells from transplant recipients. The stimulation with IL-15 promotes the CXCL12-induced migration of TEMRA and EM CD8+ T cells and promotes the generation of functional PSGL1, which interacts with the cell adhesion molecule P-selectin and adhesion of these cells to activated endothelium. Although disruption of the interaction between functional PSGL1 and P-selectin prevents the adhesion and transmigration of both TEMRA and EM CD8+ T cells, targeting VLA-4 or LFA-1 (integrins involved in T cell migration) specifically inhibited the migration of TEMRA CD8+ T cells from kidney transplant recipients. CONCLUSIONS: Our findings highlight the active role of TEMRA CD8+ T cells in humoral transplant rejection and suggest that kidney transplant recipients may benefit from therapeutics targeting these cells.

Peripheral blood immune cell profiling of acute corneal transplant rejection.

Acute rejection (AR) of corneal transplants (CT) has a profound effect on subsequent graft survival but detailed immunological studies in human CT recipients are lacking. In this multi-site, cross-sectional study, clinical details and blood samples were collected from adults with clinically diagnosed AR of full-thickness (FT)-CT (n = 35) and posterior lamellar (PL)-CT (n = 21) along with Stable CT recipients (n = 177) and adults with non-transplanted corneal disease (n = 40). For those with AR, additional samples were collected 3 months later. Immune cell analysis was performed by whole-genome microarrays (whole blood) and high-dimensional multi-color flow cytometry (peripheral blood mononuclear cells). For both, no activation signature was identified within the B cell and T cell repertoire at the time of AR diagnosis. Nonetheless, in FT- but not PL-CT recipients, AR was associated with differences in B cell maturity and regulatory CD4+ T cell frequency compared to stable allografts. These data suggest that circulating B cell and T cell subpopulations may provide insights into the regulation of anti-donor immune response in human CT recipients with differing AR risk. Our results suggest that, in contrast to solid organ transplants, genetic or cellular assays of peripheral blood are unlikely to be clinically exploitable for prediction or diagnosis of AR.

Endothelial cell, myeloid, and adaptive immune responses in SARS-CoV-2 infection.

SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) is an emerging respiratory pathogen that has rapidly spread in human populations. Severe forms of infection associate cytokine release syndrome and acute lung injury due to hyperinflammatory responses even though virus clearance is achieved. Key components of inflammation include immune cell recruitment in infected tissues, a step which is under the control of endothelial cells. Here, we review endothelial cell responses in inflammation and infection due to SARS-CoV-2 together with phenotypic and functional alterations of monocytes, T and B lymphocytes with which they interact. We surmise that endothelial cells function as an integrative and active platform for the various cells recruited, where fine tuning of immune responses takes place and which provides opportunities for therapeutic intervention.

Cochlin produced by follicular dendritic cells promotes antibacterial innate immunity.

Cochlin, an extracellular matrix protein, shares homologies with the Factor C, a serine protease found in horseshoe crabs, which is critical for antibacterial responses. Mutations in the COCH gene are responsible for human DFNA9 syndrome, a disorder characterized by neurodegeneration of the inner ear that leads to hearing loss and vestibular impairments. The physiological function of cochlin, however, is unknown. Here, we report that cochlin is specifically expressed by follicular dendritic cells and selectively localized in the fine extracellular network of conduits in the spleen and lymph nodes. During inflammation, cochlin was cleaved by aggrecanases and secreted into blood circulation. In models of lung infection with Pseudomonas aeruginosa and Staphylococcus aureus, Coch(-/-) mice show reduced survival linked to defects in local cytokine production, recruitment of immune effector cells, and bacterial clearance. By producing cochlin, FDCs thus contribute to the innate immune response in defense against bacteria.

Efficient Expansion of Human Granzyme B-Expressing B Cells with Potent Regulatory Properties.

Granzyme B-expressing B cells have been shown to be an important regulatory B cell subset in humans. However, it is unclear which subpopulations of B cells express GZMB under normal conditions and which protocols effectively induce ex vivo expansion of GZMB+ B cells. We found that in the peripheral blood of normal individuals, plasmablasts were the major B cell subpopulation that expressed GZMB. However, when using an in vitro plasmablast differentiation protocol, we obtained only 2% GZMB+ B cells. Nevertheless, using an expansion mixture containing IL-21, anti-BCR, CpG oligodeoxynucleotide, CD40L, and IL-2, we were able to obtain more than 90% GZMB+ B cells after 3 d culture. GZMB+ B cells obtained through this protocol suppressed the proliferation of autologous and allogenic CD4+CD25- effector T cells. The suppressive effect of GZMB+ B cells was partially GZMB dependent and totally contact dependent but was not associated with an increase in effector T cell apoptosis or uptake of GZMB by effector T cells. Interestingly, we showed that GZMB produced by B cells promoted GZMB+ B cell proliferation in ERK1/2-dependent manner, facilitating GZMB+ B cell expansion. However, GZMB+ B cells tended to undergo apoptosis after prolonged stimulation, which may be considered a negative feedback mechanism to limit their uncontrolled expansion. Finally, we found that expanded GZMB+ B cells exhibited a regulatory phenotype and were enriched in CD307bhi, CD258hiCD72hi, and CD21loPD-1hi B cell subpopulations. Our study, to our knowledge, provides new insight into biology of GZMB+ B cells and an efficient method to expand GZMB+ B cells for future cell therapy applications.

Cytotoxic CD8+ T lymphocytes expressing ALS-causing SOD1 mutant selectively trigger death of spinal motoneurons.

Adaptive immune response is part of the dynamic changes that accompany motoneuron loss in amyotrophic lateral sclerosis (ALS). CD4+ T cells that regulate a protective immunity during the neurodegenerative process have received the most attention. CD8+ T cells are also observed in the spinal cord of patients and ALS mice although their contribution to the disease still remains elusive. Here, we found that activated CD8+ T lymphocytes infiltrate the central nervous system (CNS) of a mouse model of ALS at the symptomatic stage. Selective ablation of CD8+ T cells in mice expressing the ALS-associated superoxide dismutase-1 (SOD1)G93A mutant decreased spinal motoneuron loss. Using motoneuron-CD8+ T cell coculture systems, we found that mutant SOD1-expressing CD8+ T lymphocytes selectively kill motoneurons. This cytotoxicity activity requires the recognition of the peptide-MHC-I complex (where MHC-I represents major histocompatibility complex class I). Measurement of interaction strength by atomic force microscopy-based single-cell force spectroscopy demonstrated a specific MHC-I-dependent interaction between motoneuron and SOD1G93A CD8+ T cells. Activated mutant SOD1 CD8+ T cells produce interferon-γ, which elicits the expression of the MHC-I complex in motoneurons and exerts their cytotoxic function through Fas and granzyme pathways. In addition, analysis of the clonal diversity of CD8+ T cells in the periphery and CNS of ALS mice identified an antigen-restricted repertoire of their T cell receptor in the CNS. Our results suggest that self-directed immune response takes place during the course of the disease, contributing to the selective elimination of a subset of motoneurons in ALS.

Transcriptional meta-analysis of regulatory B cells.

Regulatory B cells (Bregs) have the ability to regulate inflammation in various pathological situations, making them key players in immune regulation. Several mechanisms have been described and we recently identified a GZMB expressing Breg population in kidney transplanted patients who tolerate a kidney graft. To further investigate their biology and mechanisms, we conducted a transcriptomic analysis by RNAseq of these cells and we performed the first weighted meta-analysis of publicly available transcriptomic data from published Breg studies both in humans and mice. We identified two distinct and unique transcriptional signatures of 126 and 93 genes, respectively, associated with these Bregs. While we highlighted genes coding for proteins with potent involvement in regulatory functions, proliferation, and coding for transcription factors, the comparison between humans and mice did not allow identifying a common pattern. Thus, our results suggest distinct species-restricted Breg transcriptional signatures in humans and mice.

Terminally Differentiated Effector Memory CD8+ T Cells Identify Kidney Transplant Recipients at High Risk of Graft Failure.

BACKGROUND: Identifying biomarkers to predict kidney transplant failure and to define new therapeutic targets requires more comprehensive understanding of the immune response to chronic allogeneic stimulation. METHODS: We investigated the frequency and function of CD8+ T cell subsets-including effector memory (EM) and terminally differentiated EM (TEMRA) CD8+ T cells-in blood samples from 284 kidney transplant recipients recruited 1 year post-transplant and followed for a median of 8.3 years. We also analyzed CD8+ T cell reactivity to donor-specific PBMCs in 24 patients who had received living-donor kidney transplants. RESULTS: Increased frequency of circulating TEMRA CD8+ T cells at 1 year post-transplant associated with increased risk of graft failure during follow-up. This association remained after adjustment for a previously reported composite of eight clinical variables, the Kidney Transplant Failure Score. In contrast, increased frequency of EM CD8+ T cells associated with reduced risk of graft failure. A distinct TEMRA CD8+ T cell subpopulation was identified that was characterized by expression of FcγRIIIA (CD16) and by high levels of proinflammatory cytokine secretion and cytotoxic activity. Although donor-specific stimulation induced a similar rapid, early response in EM and TEMRA CD8+ T cells, CD16 engagement resulted in selective activation of TEMRA CD8+ T cells, which mediated antibody-dependent cytotoxicity. CONCLUSIONS: At 1 year post-transplant, the composition of memory CD8+ T cell subsets in blood improved prediction of 8-year kidney transplant failure compared with a clinical-variables score alone. A subpopulation of TEMRA CD8+ T cells displays a novel dual mechanism of activation mediated by engagement of the T-cell receptor or of CD16. These findings suggest that TEMRA CD8+ T cells play a pivotal role in humoral and cellular rejection and reveal the potential value of memory CD8+ T cell monitoring for predicting risk of kidney transplant failure.

Increased degradation of ATP is driven by memory regulatory T cells in kidney transplantation tolerance.

Regulatory T cells were recently proposed as the central actor in operational tolerance after renal transplantation. Tolerant patients harbor increased FoxP3hi memory Treg frequency and increased demethylation in the Foxp3 Treg-specific demethylated region when compared to stable kidney recipients and exhibit greater memory Treg suppressive capacities and higher expression of the ectonucleotidase CD39. However, in this particular and unique situation the mechanisms of action of Tregs were not identified. Thus, we analyzed the ability of memory Tregs to degrade extracellular ATP in tolerant patients, healthy volunteers, and patients with stable graft function under immunosuppression and determined the role of immunosuppressive drugs on this process. The conserved proportion of memory Tregs leads to the establishment of a pro-tolerogenic balance in operationally tolerant patients. Memory Tregs in tolerant patients display normal capacity to degrade extracellular ATP/ADP. In contrast, memory Tregs from patients with stable graft function do not have this ability. Finally, in vitro, immunosuppressive drugs may favor the lower proportion of memory Tregs in stable patients, but they have no effect on CD39-dependent ATP degradation and do not explain memory Treg lack of extracellular ATP/ADP degradation ability. Thus, intrinsic active regulatory mechanisms may act long after immunosuppressive drug arrest in operationally tolerant patients and may contribute to kidney allograft tolerance via the maintenance of CD39 Treg function.

An intermediate level of CD161 expression defines a novel activated, inflammatory, and pathogenic subset of CD8+ T cells involved in multiple sclerosis.

Several lines of evidence support a key role for CD8+ T cells in central nervous system tissue damage of patients with multiple sclerosis. However, the precise phenotype of the circulating CD8+ T cells that may be recruited from the peripheral blood to invade the CNS remains largely undefined to date. It has been suggested that IL-17 secreting CD8 (Tc17) T cells may be involved, and in humans these cells are characterized by the expression of CD161. We focused our study on a unique and recently described subset of CD8 T cells characterized by an intermediate expression of CD161 as its role in neuroinflammation has not been investigated to date. The frequency, phenotype, and function of CD8+ T cells with an intermediate CD161 expression level were characterized ex-vivo, in vitro, and in situ using RNAseq, RT-PCR, flow cytometry, TCR sequencing, and immunohistofluorescence of cells derived from healthy volunteers (n = 61), MS subjects (n = 90), as well as inflammatory (n = 15) and non-inflammatory controls (n = 6). We report here that CD8+CD161int T cells present characteristics of effector cells, up-regulate cell-adhesion molecules and have an increased ability to cross the blood-brain barrier and to secrete IL-17, IFNγ, GM-CSF, and IL-22. We further demonstrate that these cells are recruited and enriched in the CNS of MS subjects where they produce IL-17. In the peripheral blood, RNAseq, RT-PCR, high-throughput TCR repertoire analyses, and flow cytometry confirmed an increased effector and transmigration pattern of these cells in MS patients, with the presence of supernumerary clones compared to healthy controls. Our data demonstrate that intermediate levels of CD161 expression identifies activated and effector CD8+ T cells with pathogenic properties that are recruited to MS lesions. This suggests that CD161 may represent a biomarker and a valid target for the treatment of neuroinflammation.

Neuropathologic, phenotypic and functional analyses of Mucosal Associated Invariant T cells in Multiple Sclerosis.

BACKGROUND: The involvement of Mucosal Associated Invariant T (MAIT) cells, which are anti-microbial semi-invariant T cells, remains elusive in Multiple Sclerosis (MS). OBJECTIVE: Deciphering the potential involvement of MAIT cells in the MS inflammatory process. METHODS: By flow cytometry, blood MAIT cells from similar cohorts of MS patients and healthy volunteers (HV) were compared for frequency, phenotype, activation potential after in vitro TCR engagement by bacterial ligands and transmigration abilities through an in vitro model of blood-brain barrier. MS CNS samples were also studied by immunofluorescent staining and quantitative PCR. RESULTS AND CONCLUSION: Blood MAIT cells from relapsing-remitting MS patients and HV presented similar frequency, ex vivo effector phenotype and activation abilities. MAIT cells represented 0.5% of the total infiltrating T cells on 39 MS CNS lesions. This is low as compared to blood frequency (p<0.001), but consistent with their low transmigration rate. Finally, transcriptional over-expression of MR1 - which presents cognate antigens to MAIT cells - and of the activating cytokines IL-18 and IL-23 was evidenced in MS lesions, suggesting that the CNS microenvironment is suited to activate the few infiltrating MAIT cells. Taken together, these data place MAIT cells from MS patients as minor components of the inflammatory pathological process.

Peripheral phenotype and gene expression profiles of combined liver-kidney transplant patients.

BACKGROUND AND AIMS: The beneficial effect of one graft on another has been reported in combined transplantation but the associated mechanisms and biological influence of each graft have not yet been established. METHODS: In multiple analyses, we explored the PBMC phenotype and signature of 45 immune-related messenger RNAs and 754 microRNAs from a total of 235 patients, including combined liver-kidney transplant recipients (CLK), patients with a liver (L-STA) or kidney (K-STA) graft only under classical immunosuppression and patients with tolerated liver (L-TOL) or kidney grafts (K-TOL). RESULTS: CLK show an intermediary phenotype with a higher percentage of peripheral CD19(+) CD24(+) CD38(Low) memory B cells and Helios(+) Treg cells, two features associated with tolerance profiles, compared to L-STA and K-STA (P < 0.05, P < 0.01). Very few miRNA were significantly differentially expressed in CLK vs. K-STA and even fewer when compared to L-STA (35 and 8, P < 0.05). Finally, CLK are predicted to share common miRNA targets with K-TOL and even more with L-TOL (344 and 411, P = 0.005). Altogether CLK display an intermediary phenotype and gene profile, which is closer to that of liver transplant patients, with possible similarities with the profiles of tolerant patients. CONCLUSION: These data suggest that CLK patients show the immunological influence of both allografts with liver having a greater influence.

Central Role of CD45RA- Foxp3hi Memory Regulatory T Cells in Clinical Kidney Transplantation Tolerance.

The role of Foxp3(+) regulatory T cells (Tregs) in operational tolerance remains elusive, as initial results revealed an increased frequency of this subset in tolerant patients but no functional differences compared with immunosuppressed recipients. In addition, recent studies of regulatory B cells strongly suggest that Tregs may not have a central role in kidney transplantation tolerance. However, recent investigations of the crucial role of Foxp3 demethylation in Treg function and the possibility of identifying distinct Foxp3 T cell subsets prompted us to more thoroughly characterize Tregs in operationally tolerant patients. Thus, we studied the level of demethylation of the Foxp3 Treg-specific demethylated region (TSDR) in circulating CD4(+) T cells and analyzed Treg subset frequency in tolerant patients, healthy volunteers, patients with stable graft function under immunosuppression, and chronically rejecting recipients. We observed a higher proportion of CD4(+) T cells with demethylated Foxp3 and a specific expansion of CD4(+) CD45RA(-) Foxp3(hi) memory Tregs exclusively in tolerant patients. The memory Tregs of tolerant recipients exhibited increased Foxp3 TSDR demethylation, expressed higher levels of CD39 and glucocorticoid-induced TNF-related receptor, and harbored greater suppressive properties than memory Tregs from patients with stable graft function. Taken together, our data demonstrate that operationally tolerant patients mobilize an array of potentially suppressive cells, including not only regulatory B cells but also Tregs. Our results also indicate that tolerant patients have potent CD4(+)CD45RA(-) Foxp3(hi) memory Tregs with a specific Foxp3 TSDR demethylation pattern, which may contribute to the maintenance of graft tolerance.

Tolerant Kidney Transplant Patients Produce B Cells with Regulatory Properties.

Whereas a B cell-transcriptional profile has been recorded for operationally tolerant kidney graft patients, the role that B cells have in this tolerance has not been reported. In this study, we analyzed the role of B cells from operationally tolerant patients, healthy volunteers, and kidney transplant recipients with stable graft function on T cell suppression. Proliferation, apoptosis, and type I proinflammatory cytokine production by effector CD4(+)CD25(-) T cells were measured after anti-CD3/anti-CD28 stimulation with or without autologous B cells. We report that B cells inhibit CD4(+)CD25(-) effector T cell response in a dose-dependent manner. This effect required B cells to interact with T-cell targets and was achieved through a granzyme B (GzmB)-dependent pathway. Tolerant recipients harbored a higher number of B cells expressing GzmB and displaying a plasma cell phenotype. Finally, GzmB(+) B-cell number was dependent on IL-21 production, and B cells from tolerant recipients but not from other patients positively regulated both the number of IL-21(+) T cells and IL-21 production, suggesting a feedback loop in tolerant recipients that increases excessive B cell activation and allows regulation to take place. These data provide insights into the characterization of B cell-mediated immunoregulation in clinical tolerance and show a potential regulatory effect of B cells on effector T cells in blood from patients with operationally tolerant kidney grafts.

A common gene signature across multiple studies relate biomarkers and functional regulation in tolerance to renal allograft.

Patients tolerant to a kidney graft display a specific blood cell transcriptional pattern but results from five different studies were inconsistent, raising the question of relevance for future clinical application. To resolve this, we sought to identify a common gene signature, specific functional and cellular components, and discriminating biomarkers for tolerance following kidney transplantation. A meta-analysis of studies identified a robust gene signature involving proliferation of B and CD4 T cells, and inhibition of CD14 monocyte related functions among 96 tolerant samples. This signature was further supported through a cross-validation approach, yielding 92.5% accuracy independent of the study of origin. Experimental validation, performed on new tolerant samples and using a selection of the top-20 biomarkers, returned 91.7% of good classification. Beyond the confirmation of B-cell involvement, our data also indicated participation of other cell subsets in tolerance. Thus, the use of the top 20 biomarkers, mostly centered on B cells, may provide a common and standardized tool towards personalized medicine for the monitoring of tolerant or low-risk patients among kidney allotransplant recipients. These data point to a global preservation of genes favoring the maintenance of a homeostatic and 'healthy' environment in tolerant patients and may contribute to a better understanding of tolerance maintenance mechanisms.

Expansion of highly differentiated cytotoxic terminally differentiated effector memory CD8+ T cells in a subset of clinically stable kidney transplant recipients: a potential marker for late graft dysfunction.

Despite the effectiveness of immunosuppressive drugs, kidney transplant recipients still face late graft dysfunction. Thus, it is necessary to identify biomarkers to detect the first pathologic events and guide therapeutic target development. Previously, we identified differences in the T-cell receptor Vß repertoire in patients with stable graft function. In this prospective study, we assessed the long-term effect of CD8(+) T-cell differentiation and function in 131 patients who had stable graft function. In 45 of 131 patients, a restriction of TCR Vß diversity was detected and associated with the expansion of terminally differentiated effector memory (TEMRA; CD45RA(+)CCR7(-)CD27(-)CD28(-)) CD8(+) T cells expressing high levels of perforin, granzyme B, and T-bet. This phenotype positively correlated with the level of CD57 and the ability of CD8(+) T cells to secrete TNF-α and IFN-γ. Finally, 47 of 131 patients experienced kidney dysfunction during the median 15-year follow-up period. Using a Cox regression model, we found a 2-fold higher risk (P=0.06) of long-term graft dysfunction in patients who had increased levels of differentiated TEMRA CD8(+) T cells at inclusion. Collectively, these results suggest that monitoring the phenotype and function of circulating CD8(+) T cells may improve the early identification of at-risk patients.

Unaltered regulatory B-cell frequency and function in patients with multiple sclerosis.

Multiple sclerosis (MS) is a chronic disease of the central nervous system (CNS) typically characterized by the recruitment of T cells into the CNS. However, certain subsets of B cells have been shown to negatively regulate autoimmune diseases and some data support a prominent role for B cells in MS physiopathology. For B cells in MS patients we analyzed subset frequency, cytokine secretion ability and suppressive properties. No differences in the frequencies of the B-cell subsets or in their ability to secrete cytokines were observed between MS and healthy volunteers (HV). Prestimulated B cells from MS patients also inhibited CD4(+)CD25(-) T cell proliferation with a similar efficiency as B cells from HV. Altogether, our data show that, in our MS patient cohort, regulatory B cells have conserved frequency and function.

Blood Gene Signature as a Biomarker for Subclinical Kidney Allograft Rejection: Where Are We?

The observation decades ago that inflammatory injuries because of an alloimmune response might be present even in the absence of concomitant clinical impairment in allograft function conduced to the later definition of subclinical rejection. Many studies have investigated the different subclinical rejections defined according to the Banff classification (subclinical T cell-mediated rejection and antibody-mediated rejection), overall concluding that these episodes worsened long-term allograft function and survival. These observations led several transplant teams to perform systematic protocolar biopsies to anticipate treatment of rejection episodes and possibly prevent allograft loss. Paradoxically, the invasive characteristics and associated logistics of such procedures paved the way to investigate noninvasive biomarkers (urine and blood) of subclinical rejection. Among them, several research teams proposed a blood gene signature developed from cohort studies, most of which achieved excellent predictive values for the occurrence of subclinical rejection, mainly antibody-mediated rejection. Interestingly, although all identified genes relate to immune subsets and pathways involved in rejection pathophysiology, very few transcripts are shared among these sets of genes, highlighting the heterogenicity of such episodes and the difficult but mandatory need for external validation of such tools. Beyond this, their application and value in clinical practice remain to be definitively demonstrated in both biopsy avoidance and prevention of clinical rejection episodes. Their combination with other biomarkers, either epidemiological or biological, could contribute to a more accurate picture of a patient's risk of rejection and guide clinicians in the follow-up of kidney transplant recipients.

Fine-tuning tumor- and allo-immunity: advances in the use of immune checkpoint inhibitors in kidney transplant recipients.

Cancer is a common complication after kidney transplantation. Kidney transplant recipients (KTR) have a 2- to 4-fold higher risk of developing cancer compared to the general population and post-transplant malignancy is the third most common cause of death in KTR. Moreover, it is well known that certain cancer types are overrepresented after transplantation, especially non-melanoma skin cancer. Immune checkpoint inhibitors (ICI) have revolutionized the treatment of cancer, with remarkable survival benefit in a subgroup of patients. ICI are monoclonal antibodies that block the binding of specific co-inhibitory signaling molecules. Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1), and its ligand programmed cell death ligand 1 (PD-L1) are the main targets of ICI. Solid organ transplant recipients (SOTR) have been excluded from clinical trials owing to concerns about tumor response, allo-immunity, and risk of transplant rejection. Indeed, graft rejection has been estimated as high as 48% and represents an emerging problem. The underlying mechanisms of organ rejection in the context of treatment with ICI are poorly understood. The search for restricted antitumoral responses without graft rejection is of paramount importance. This review summarizes the current knowledge of the use of ICI in KTR, the potential mechanisms involved in kidney graft rejection during ICI treatment, potential biomarkers of rejection, and how to deal with rejection in clinical practice.