The typical course of HIV infection for a majority of untreated individuals is persistent viral replication and a gradual loss of CD4 + T cells. One of the consequences of ongoing HIV replication is increased immune activation, aff ecting all major cell populations of the immune system ( 1 -3 ). Within the B cell population, HIV infection has been associated with numerous perturbations ( 4 ), many of which have been attributed to changes in the distribution of B cell subpopulations found in the peripheral blood. These changes include increased frequencies of activated and terminally diff erentiated B cells expressing low levels of CD21 that have been associated with ongoing viral replication ( 5, 6 ), a decreased frequency of memory B cells that is not reversed by antiretroviral therapy ( 7 ), and an increased frequency of immature/transitional B cells that has been associated with CD4 + T cell lymphopenia ( 8, 9 ).The eff ects of immune activation in persistent viral infections have recently been shown to include virus-specifi c T cell exhaustion. After the original description in chronic lymphocyte choriomeningitis virus (LCMV) infection in mice ( 10 ), observations of virus-specifi c CD4 + and CD8 + T cell exhaustion have recently been extended to 12 ). Although PD-1 was the fi rst inhibitory receptor associated with virus-specifi c T cell exhaustion, recent fi ndings suggest that exhaustion may result

Background Mendelian analysis of disorders of immune regulation can provide insight into molecular pathways associated with host defense and immune tolerance. Methods We identified three families with a dominantly inherited complex of cold-induced urticaria, antibody deficiency, and susceptibility to infection and autoimmunity. Immunophenotyping methods included flow cytometry, analysis of serum immunoglobulins and autoantibodies, lymphocyte stimulation, and enzymatic assays. Genetic studies included linkage analysis, targeted Sanger sequencing, and next-generation whole-genome sequencing. Results Cold urticaria occurred in all affected subjects. Other, variable manifestations included atopy, granulomatous rash, autoimmune thyroiditis, the presence of antinuclear antibodies, sinopulmonary infections, and common variable immunodeficiency. Levels of serum IgM and IgA and circulating natural killer cells and class-switched memory B cells were reduced. Linkage analysis showed a 7-Mb candidate interval on chromosome 16q in one family, overlapping by 3.5 Mb a disease-associated haplotype in a smaller family. This interval includes PLCG2, encoding phospholipase Cγ2 (PLCγ2), a signaling molecule expressed in B cells, natural killer cells, and mast cells. Sequencing of complementary DNA revealed heterozygous transcripts lacking exon 19 in two families and lacking exons 20 through 22 in a third family. Genomic sequencing identified three distinct in-frame deletions that cosegregated with disease. These deletions, located within a region encoding an autoinhibitory domain, result in protein products with constitutive phospholipase activity. PLCG2-expressing cells had diminished cellular signaling at 37°C but enhanced signaling at subphysiologic temperatures. Conclusions Genomic deletions in PLCG2 cause gain of PLCγ2 function, leading to signaling abnormalities in multiple leukocyte subsets and a phenotype encompassing both excessive and deficient immune function. (Funded by the National Institutes of Health Intramural Research Programs and others.)

IntroductionNumerous perturbations of B-cell phenotype and function have been described in HIV-infected individuals (reviewed in Cagigi et al 1 and Moir and Fauci 2 ). Phenotypic perturbations of B cells circulating in the peripheral blood include over-representation of activated, exhausted, and terminally differentiated B cells associated with HIV viremia 3-5 ; over-representation of immature/ transitional B cells associated with HIV-induced CD4 ϩ T-cell lymphopenia 6,7 ; and reduced representation of CD27 ϩ memory B cells associated with most stages of HIV infection. [8][9][10][11][12][13] Most of these studies, whether longitudinal or cross-sectional, have been conducted on chronically HIV-infected individuals, whereas only a few studies, generally with small sample sizes, have addressed the effect of duration of infection and initiation of antiretroviral therapy (ART) on perturbations of B-cell subpopulations in HIVinfected individuals. 9,14,15 Functional perturbations of B cells in HIV-infected individuals include hypergammaglobulinemia associated with polyclonal and HIV-specific activation of B cells induced by ongoing HIV replication, 15,16 as well as decreased B-cell responses to specific immunogens and non-HIV pathogens. 11,17,18 The latter is likely a reflection of both CD4 ϩ T-cell dependent and independent defects in B cells that arise in HIV-infected individuals, and especially in individuals with ongoing viral replication. Many of the functional B-cell defects described in HIV-viremic individuals can be improved with ART, although there is 1 important exception that has received relatively little attention. While B-cell responses against non-HIV antigens are either stabilized or increased after initiation of ART, 19-22 the reverse is often observed for B-cell responses against HIV antigens,15,23,24 suggesting that the humoral response against HIV is dependent on continuous HIV replication. In the most comprehensive study on B-cell responses after ART, Morris and colleagues described a rapid loss of HIV-specific B cells (actively secreting plasmablasts) during therapy, followed by a more gradual decrease in antibody titers against HIV in chronically infected individuals; the loss was even more rapid in early-treated individuals. 15 In a previous study of chronically HIV-infected individuals, we reported a reduction in B-cell numbers and the presence of perturbed B-cell subpopulations before initiation of ART followed by partial normalization 1 year after successful reduction in viremia by ART. 25 In a more recent study, 5 we identified a subpopulation of CD27 Ϫ tissue-like memory B cells within an abnormal CD21 lo B-cell compartment of the peripheral blood of chronically HIVviremic individuals; this subpopulation had been included within the activated B-cell compartment that we had previously described. 25 These CD27 Ϫ tissue-like memory B cells bore many features of exhausted cells, arising in the context of chronic immune activation, and with features that include expression of multiple inhibitory...

Progression of HIV disease is associated with the appearance of numerous B cell defects. We describe herein a population of immature͞transitional B cells that is overly represented in the peripheral blood of individuals with advancing HIV disease. These B cells, identified by the expression of CD10, were unresponsive by proliferation to B cell receptor triggering and possessed a phenotype and an Ig diversity profile that confirmed their immature͞ transitional stage of differentiation. Consistent with an immature status, their lack of proliferation to B cell receptor triggering was reversed with CD40 ligand, but not B cell activation factor. Finally, levels of CD10 expression on B cells were directly correlated with serum levels of IL-7, suggesting that increased levels of IL-7 modulate human B cell maturation either directly or indirectly by means of a homeostatic effect on lymphopenia. Taken together, these data offer insight into human B cell development as well as B cell dysfunction in advanced HIV disease that may be linked to IL-7-dependent homeostatic events.immunopathogenesis ͉ CD10 ͉ CD21 ͉ B cell receptor ͉ lymphopenia