CD9 belongs to the tetraspanin superfamily. Depending on the cell type and associated molecules, CD9 has a wide variety of biological activities such as cell adhesion, motility, metastasis, growth, signal transduction, differentiation, and sperm–egg fusion. This review focuses on CD9 expression by hematopoietic cells and its role in modulating cellular processes involved in the regulation of inflammation. CD9 is functionally very important in many diseases and is involved either in the regulation or in the mediation of the disease. The role of CD9 in various diseases, such as viral and bacterial infections, cancer and chronic lung allograft dysfunction, is discussed. This review focuses also on its interest as a biomarker in diseases. Indeed CD9 is primarily known as a specific exosome marker however, its expression is now recognized as an anti-inflammatory marker of monocytes and macrophages. It was also described as a marker of murine IL-10-competent Breg cells and IL-10-secreting CD9⁺ B cells were associated with better allograft outcome in lung transplant patients, and identified as a new predictive biomarker of long-term survival. In the field of cancer, CD9 was both identified as a favorable prognostic marker or as a predictor of metastatic potential depending on cancer types. Finally, this review discusses strategies to target CD9 as a therapeutic tool. Because CD9 can have opposite effects depending on the situation, the environment and the pathology, modulating CD9 expression or blocking its effects seem to be a new promising therapeutic strategy.

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.

Operationally tolerant patients (TOL) display a higher number of blood B cells and transcriptional B cell signature. As they rarely develop an allo‐immune response, they could display an abnormal B cell differentiation. We used an in vitro culture system to explore T‐dependent differentiation of B cells into plasma cells. B cell phenotype, apoptosis, proliferation, cytokine, immunoglobulin production and markers of differentiation were followed in blood of these patients. Tolerant recipients show a higher frequency of CD20⁺CD24^hiCD38^hi transitional and CD20⁺CD38^loCD24^lo naïve B cells compared to patients with stable graft function, correlating with a decreased frequency of CD20⁻CD38⁺CD138⁺ differentiated plasma cells, suggestive of abnormal B cell differentiation. B cells from TOL proliferate normally but produce more IL‐10. In addition, B cells from tolerant recipients exhibit a defective expression of factors of the end step of differentiation into plasma cells and show a higher propensity for cell death apoptosis compared to patients with stable graft function. This in vitro profile is consistent with down‐regulation of B cell differentiation genes and anti‐apoptotic B cell genes in these patients in vivo. These data suggest that a balance between B cells producing IL‐10 and a deficiency in plasma cells may encourage an environment favorable to the tolerance maintenance.

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.

Chronic rejection (CR) is a major cause of long-term graft loss that would be avoided by the induction of tolerance. We previously showed that renal transplant patients with CR have lower numbers of peripheral CD4⁺CD25^high T cells than operationally tolerant patients, patients with stable graft function and healthy volunteers (HV). We explored here the profile of CD4⁺CD25^high blood T cells in these patients focusing on their expression of the regulatory T cells (Treg) gene Forkhead Box P3 (FOXP3) and their suppressive function. We show that CR is associated with a decreased number of CD4⁺CD25^highFOXP3⁺T cells with normal regulatory profile, whereas graft acceptance is associated with CD4⁺CD25^highFOXP3⁺T cell numbers similar to HVs. These data suggest that Treg numbers, rather than their intrinsic suppressive capacity, may contribute to determining the long-term fate of renal transplants.