Key Points• Deficiency of TPP2 is associated with Evans syndrome and viral infection susceptibility.• TPP2 deficiency links premature immunosenescence of T and B cells with severe autoimmunity.Autoimmune cytopenia is a frequent manifestation of primary immunodeficiencies. Two siblings presented with Evans syndrome, viral infections, and progressive leukopenia. DNA available from one patient showed a homozygous frameshift mutation in tripeptidyl peptidase II (TPP2) abolishing protein expression. TPP2 is a serine exopeptidase involved in extralysosomal peptide degradation. Its deficiency in mice activates cell death programs and premature senescence. Similar to cells from naïve, uninfected TPP2-deficient mice, patient cells showed increased major histocompatibility complex I expression and most CD8 1 T-cells had a senescent CCR7-CD127 2 CD28 2 CD57 1 phenotype with poor proliferative responses and enhanced staurosporine-induced apoptosis. T-cells showed increased expression of the effector molecules perforin and interferon-g with high expression of the transcription factor T-bet. Age-associated B-cells with a CD212 CD11c 1 phenotype expressing T-bet were increased in humans and mice, combined with antinuclear antibodies. Moreover, markers of senescence were also present in human and murine TPP2-deficient fibroblasts.Telomere lengths were normal in patient fibroblasts and granulocytes, and low normal in lymphocytes, which were compatible with activation of stress-induced rather than replicative senescence programs. TPP2 deficiency is the first primary immunodeficiency linking premature immunosenescence to severe autoimmunity. Determination of senescent lymphocytes should be part of the diagnostic evaluation of children with refractory multilineage cytopenias. (Blood. 2015;125(5):753-761)

Familial hemophagocytic lymphohistiocytosis (FHL) is a hyperinflammatory syndrome affecting patients with genetic cytotoxicity defects. Perforin‐deficient (PKO) mice recapitulate the full clinical picture of FHL after infection with lymphocytic choriomeningitis virus (LCMV). Hyperactivated CD8+ T cells and IFN‐γ have been identified as the key drivers of FHL and represent targets for therapeutic interventions. However, the response of patients is variable. This could be due to trigger‐dependent differences in pathogenesis, which is difficult to address in FHL patients, since the trigger frequently escapes detection. We established an alternative FHL model using intravenous infection of PKO mice with murine CMV (MCMV)Smith. PKO mice developed acute FHL after both infections and fulfilled HLH diagnostic criteria accompanied by excessive IFN‐γ production by disease‐inducing T cells, that enrich in the BM. However, direct comparison of the two infection models disclosed trigger‐dependence of FHL progression and revealed a higher contribution of CD4 T cells and NK cells to IFN‐γ production after MCMV infection. Importantly, therapeutic intervention by IFN‐γ neutralization or CD8+ T‐cell depletion had less benefit in MCMV‐triggered FHL compared to LCMV‐triggered FHL, likely due to MCMV‐induced cytopathology. Thus, the context of the specific triggering viral infection can impact the success of targeted immunotherapeutic HLH control.

Defining the minimal thresholds for effective antiviral T cell immunity is important for clinical decisions in immunodeficient patients. TCR signaling is critical for T cell development, activation, and effector functions. In this article, we analyzed which of these TCR-mediated processes is limiting for antiviral immunity in a mouse strain with reduced expression of SLP-76 (twp mice). Despite severe T cell activation defects in vitro, twp mice generated a normal proportion of antiviral effector T cells postinfection with lymphocytic choriomeningitis virus (LCMV). Twp CD8(+) T cells showed impaired polyfunctional cytokine production, whereas cytotoxicity as the crucial antiviral effector function for LCMV control was normal. The main limiting factor in the antiviral response of twp mice was impaired T cell proliferation and survival, leading to a 5- to 10-fold reduction of antiviral T cells at the peak of the immune response. This was still sufficient to control infection with the LCMV Armstrong strain, but the more rapidly replicating LCMV-WE induced T cell exhaustion and viral persistence. Thus, under conditions of impaired TCR signaling, reduced T cell expansion was the limiting factor in antiviral immunity. These findings have implications for understanding antiviral immunity in patients with T cell deficiencies.