None of the patients received steroids or immune modulating drugs at the time of sampling. comparable with HIV patients with decreased naive CD4+ and CD8+ T cells and increased effector CD8+ T cells. Like in HIV+ patients, expression of activation markers and inhibitory receptors CD160, CD244, and programmed death receptor (PD)-1 on CD8+ T cells was increased in APDS patients, indicating exhaustion. EBV-specific CD8+ T cells from APDS patients exhibited an exhausted phenotype that resembled HIV-specific HGFR CD8+ T cells in terms of inhibitory receptor expression. Inhibition of PD-1 on EBV-specific CD8+ T cells from APDS patients enhanced proliferation and effector cytokine production. Based on these results, we conclude that total and EBV-specific CD8+ T cells from APDS patients are characterized by T cell exhaustion. Furthermore, PD-1 checkpoint inhibition may provide a possible therapeutic approach to support the immune system of APDS patients to control EBV and CMV. (41, 42, 44C46) and (40, 47C51). In addition, inhibitory receptor blockade was introduced into the clinic to re-activate exhausted T cells in cancer (52, 53). Previously, total and virus-specific CD8+ T cells in APDS patients were shown to have upregulated CD57 expression and reduced proliferative capacity. These findings were interpreted as T cell senescence (12, 54C56). However, patients lymphocytes also exhibited an increased rate of apoptosis compared with healthy controls (11, 15, 54), which LY 254155 is not in line with the resistance to apoptosis that has been ascribed to T cell senescence (21). Increased apoptosis sensitivity is usually associated with exhaustion rather than senescence (39, 57). In addition, APDS patient T cells have been reported to express more PD-1, a receptor associated with T cell activation and exhaustion (12, 54, 55). The PI3K pathway is critical for TCR signaling in CD8+ T cells (58) and chronic antigen stimulation alone is sufficient to lead to CD8+ T cell exhaustion (59). This raises the question whether GOF mutations LY 254155 in PI3K lead to changes in the activation of T cells which might predispose for T cell exhaustion rather than or in addition to immune senescence. Understanding the mechanisms leading to impaired immune response in APDS patients is a requirement to define the best treatment options for these patients that can support the control of viral infections, which could in turn reduce virus-related morbidities in these patients. To elucidate the role of exhaustion in APDS patients, total CD8+ T cells and CD4+ T cells from APDS patients were phenotypically characterized and compared with T cells from healthy individuals and HIV-infected patients (HIV+ patients). We have included peripheral blood mononuclear cells (PBMC) from HIV-infected patients since it is usually well established that HIV contamination leads to exhaustion of HIV-specific CD8+ T cells but not CMV-specific CD8+ T cells in HIV-infected patients (39, 41) and can therefore serve as a positive control for exhaustion. Furthermore, virus-specific CD8+ T cells in all three groups were characterized, and the effect of PD-1 blockade on proliferation and effector functions was investigated. Our findings indicate that indeed CD8+ T cells from APDS patients are more similar to the ones from HIV+ patients and exhibit characteristics of exhaustion. Importantly, we show that blocking PD-1 signaling can increase virus-specific CD8+ T cell proliferation and cytokine production. Our findings suggest that CD8+ T cells in APDS patients undergo exhaustion, and that this may contribute to the impaired control of persistent viral infections such as EBV and CMV. These findings raise the LY 254155 possibility of checkpoint inhibition as a treatment strategy to support APDS patients to control recurrent or chronic viral infections. Materials and Methods Cell Samples and Ethical Approval This study was carried out in accordance with the recommendations of Erasmus MC Medical Ethics Committee with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the Erasmus MC Medical Ethics Committee. Ten APDS patients were included with a median age of 27?years (range 6C44?years), and a gender ratio of six males to four females. Nine of the patients LY 254155 have a mutation in the PIK3CD gene (seven patients: E1021K mutation, one patient: S312R mutation, and one patient: R929C mutation) (15), and one patient has.