Vaccine-induced plasmablast responses in rhesus macaques: phenotypic characterization and a source for generating antigen-specific monoclonal antibodies.

Over 100 broadly neutralizing antibodies have been isolated from a minority of HIV infected patients, but the steps leading to the selection of plasma cells producing such antibodies remain incompletely understood, hampering the development of vaccines able to elicit them. Rhesus macaques have become a preferred animal model system used to study SIV/HIV, for the characterization and development of novel therapeutics and vaccines as well as to understand pathogenesis. However, most of our knowledge about the dynamics of antibody responses is limited to the analysis of serum antibodies or monoclonal antibodies generated from memory B cells. In a vaccine setting, relatively little is known about the early cellular responses that elicit long-lived plasma cells and memory B cells and the tools to dissect plasmablast responses are not available in macaques. In the current study, we show that the majority (>80%) of the vaccine-induced plasmablast response are antigen-specific by functional ELISPOT assays. While plasmablasts are easily defined and isolated in humans, those same phenotypic markers have not been useful for identifying macaque plasmablasts. Here we describe an approach that allows for the isolation and single cell sorting of vaccine-induced plasmablasts. Finally, we show that isolated plasmablasts can be used to efficiently recover antigen-specific monoclonal antibodies through single cell expression cloning. This will allow detailed studies of the early plasmablast responses in rhesus macaques, enabling the characterization of both their repertoire breadth as well as the epitope specificity and functional qualities of the antibodies they produce, not only in the context of SIV/HIV vaccines but for many other pathogens/vaccines as well.

PD-L1 blockade synergizes with IL-2 therapy in reinvigorating exhausted T cells.

The inhibitory receptor programmed cell death 1 (PD-1) plays a major role in functional exhaustion of T cells during chronic infections and cancer, and recent clinical data suggest that blockade of the PD-1 pathway is an effective immunotherapy in treating certain cancers. Thus, it is important to define combinatorial approaches that increase the efficacy of PD-1 blockade. To address this issue, we examined the effect of IL-2 and PD-1 ligand 1 (PD-L1) blockade in the mouse model of chronic lymphocytic choriomeningitis virus (LCMV) infection. We found that low-dose IL-2 administration alone enhanced CD8+ T cell responses in chronically infected mice. IL-2 treatment also decreased inhibitory receptor levels on virus-specific CD8+ T cells and increased expression of CD127 and CD44, resulting in a phenotype resembling that of memory T cells. Surprisingly, IL-2 therapy had only a minimal effect on reducing viral load. However, combining IL-2 treatment with blockade of the PD-1 inhibitory pathway had striking synergistic effects in enhancing virus-specific CD8+ T cell responses and decreasing viral load. Interestingly, this reduction in viral load occurred despite increased numbers of Tregs. These results suggest that combined IL-2 therapy and PD-L1 blockade merits consideration as a regimen for treating human chronic infections and cancer.

Follicular B helper T cell activity is confined to CXCR5(hi)ICOS(hi) CD4 T cells and is independent of CD57 expression.

The generation of high-affinity antibody-secreting plasma cells critically depends on the presence of CD4 T cells during the germinal center (GC) reaction. GC T cells are so far incompletely characterized in terms of phenotype and function. Here, we show that human follicular B helper T (T(FH)) cells are characterized by high expression of the homeostatic chemokine receptor CXCR5 and the costimulatory molecule ICOS, but not CD57 expression. CXCR5(hi)ICOS(hi) CD4 T cells are the most potent inducers of IgG production that also secrete large amounts of the B cell-attracting chemokine CXCL13. CXCR5(hi)ICOS(hi) CD4 T cells differ from other tonsillar CD4 T cell subsets in their stimulatory activity, proliferative capacity and susceptibility to apoptosis. Large-scale gene expression analysis revealed that T(FH) cells are only distantly related to CXCR5(-) and CXCR5(+) central memory T (T(CM)) as well as effector memory T (T(EM)) cells present in the periphery. CXCR5(hi)ICOS(hi) CD4 T cells appear to be terminally differentiated T helper cells that express a unique set of transcription factors related to the Notch signaling pathway and thus differentiate independent of other T helper cell populations.