Background. Class switch DNA recombination (CSR) from IgM to IgG and IgA is central to immunity against pathogens and requires the activation of B cells by CD4+ T cells through CD40 ligand (CD40L) and cytokines, including IL-4 and IL-10. Viruses evade protective IgG and IgA responses through a number of strategies, including production of soluble proteins that suppress T cell-dependent (TD) class switching in bystander B cells. HIV-1 is thought to impair antigen-specific IgG and IgA production by inducing progressive depletion of CD4+ T cells. Although important, this mechanism is not sufficient to explain the intrinsically poor response of B cells from viremic patients to CD4+ T cell help. This prompted us to explore the B cell-modulating activity of negative factor (Nef), an early HIV-1 protein implicated in immunosuppression. Nef is released in the extracellular environment by infected cells and might perturb the function of bystander B cells by targeting them through its N-terminal myristoylated domain.
Methods. Tonsillar tissue sections from healthy donors and lymph nodal tissue sections from 10 HIV-1-infected patients were analyzed for IgD, p24, and Nef expression by immunohistochemistry. Nef expression was also analyzed in IgD+ B cells purified from healthy donors and exposed to a recombinant myristoylated or non-myristoylated Nef protein. These IgD+ B cells as well as an IgD+ B cell line expressing a wild-type Nef transgene or a mutant Nef transgene lacking the myristoylated domain were utilized in CSR assays after further exposure to CD40L, IL-4 and IL-10. CSR and signaling were studied as described1,2,3.
Results. We found that HIV-1-infected lymphoid follicles express large amounts of Nef. This viral protein penetrates in bystander B cells both in vivo and in vitro and interferes with the initiation of CSR by CD40L and cytokines without down-regulating CD40 and cytokine receptor expression on B cells. Rather, Nef up-regulates inhibitor of NF-κ B (Iκ B) and suppressor of cytokine signaling (SOCS) proteins, including SOCS1 and SOCS3, in B cells. These powerful inhibitory proteins attenuate the transcriptional activation of germline Ig gene promoters by blocking CD40 and cytokine receptor signaling through NF-κ B and signal transducer and activator of transcription (STAT), respectively. In addition, Nef hampers the up-regulation of key components of the CSR machinery, such as activation-induced cytidine deaminase (AID), and blocks the induction of IgG, IgA and IgE secretion by CD40L and cytokines. Under similar conditions, Nef spares B cell survival, B cell proliferation as well as B cell signaling through mitogen-activated protein kinase p38 and B cell-specific activation protein (BSAP or Pax5).
Conclusions. Our findings suggest that HIV-1 inhibits IgG and IgA production not only by impairing CD4+ T cells, but also by turning on powerful CSR inhibitory pathways in bystander B cells through Nef. We propose that Nef-blocking agents might improve protective antibody responses to pathogens and vaccines in HIV-1-infected patients.