Impairment of B cell trafficking and differentiation during HIV-1 infection

Abstract

During the course of HIV-1 infection, several B cell dysfunctions occur as result of virus replication and indirect mechanisms of immunopathology. The B cells abnormalities include hypergammaglobulinemia, a decreased number of memory B cells, increased levels of activation markers on B cell surface and plasmacytosis. The molecular bases for these impairments are not fully characterized but may have relevance for designing functional HIV vaccines and improved treatment.

In paper I the expression of chemokine receptors/chemokines important for B cell function was determined on cells from HIV-1 infected patients and controls. We studied the CXC chemokine receptor 4 (CXCR4), CXCR5, and CC chemokine receptor 7 (CCR7) and their respective ligands. We found a decreased expression of CXCR5 to be present on blood B cells from patients (P<0.05), in association with low CD4+ T-cell counts. Interestingly, B cells in blood and lymph nodes from HIV-1-infected patients also displayed an increased expression of the CXC chemokine ligand 13 (CXCL13), the ligand for CXCR5. Upon B-cell activation in vitro, CXCL13 was secreted in culture. The findings suggest that altered CXCR5/CXCL13 expression may participate in B-cell dysfunctions during HIV-1 infection. Loss of memory B cells is a regular finding in the blood of HIV-1 infected patients and the possibility exists that increased apoptosis via the Fas death receptor pathway may participate in this pathological mechanism. Interleukin-7 (IL-7), present to high levels in blood of HIV-1 infected patients, was previously reported to lead to increased Fas expression and Fas mediated apoptosis on T cells.

In paper II, a novel mechanism responsible for increased B cell apoptosis in presence of the high IL-7 concentration was described. T cells cultured with IL-7 induced high Fas expression on resting B cells together with an increased sensitivity to Fas mediated apoptosis. As the mediator responsible for B cell priming to Fas mediated apoptosis we identified the cytokine IFN-γ that T cells secrete in response to IL-7. These results indicate a potential link between IL-7 and the increased B cell apoptosis in HIV-1 infected individuals. During HIV-1 infection, loss of memory B-cells, together with an altered differentiation of naïve B-cells, result in production of low quality antibodies, which may be due to impaired immunoglobulin affinity maturation.

In paper III, we evaluated the effect of HIV-1 infection on class switch recombination and somatic hypermutation, crucial processes for the generation of functional antibodies, by studying the expression of activation-induced cytidine deaminase (AID) in peripheral B-cells from HIV-1 infected patients and healthy controls. We also studied the phenotype of B cells and their ability to produce immunoglobulins in vitro. Cells from HIV-1 infected patients showed higher baseline levels of AID expression and increased IgA production measured ex-vivo and upon CD40 and TLR9 stimulation in vitro. Moreover, the percentage of CD27(-)IgA+ and CD27(-)IgG+ B-cells in blood was significantly increased in HIV-1 infected patients. Interestingly, our results also showed a significantly increased number of somatic hypermutations in the VH genes in CD27(-) cells from patients.

Taken together, the results show that during HIV-1 infection, CD27(-) B-cells can produce class switched and somatically hypermutated antibodies. High levels of soluble CD27 (sCD27), a marker of immune activation, are found during HIV-1 infection; whether sCD27 has a biological role on B cells was previously not known. The aim of paper IV was to investigate whether sCD27, by binding to CD70, can induce IgG production from B cells. B cells from healthy and HIV-1-infected individuals were cultured with recombinant human sCD27 (rhsCD27) and IgG production was measured in culture. We demonstrated that rhsCD27 induced IgG production from antigen-primed (CD27+) B cells. This effect was mediated by rhsCD27 binding to CD70 on B cells leading to activation of Blimp-1 and XBP-1, transcription factors associated with plasma cell differentiation. We found a significant correlation between the levels of serum sCD27 and IgG in HIV-1-infected individuals and healthy controls. The sCD27 may act to enhance immunoglobulin production and differentiation of activated memory B cells, thus providing an activation signal to antigen-experienced B cells. This mechanism may operate during HIV-1 infection when continuous immune activation may lead to up-regulation of CD70 expression and increased sCD27 cleavage and account for increased levels of circulating IgG.

In conclusion, in this PhD thesis different mechanisms leading to impairments of B cell function observed during HIV-1 infection, in parallel to abnormal events of immune activation, are characterized.