Highlights d Slow delivery immunization enhances HIV neutralizing antibody development in monkeys d Slow delivery immunization alters immunodominance of the responding B cells d Weekly longitudinal germinal center (GC) B and T FH analyses provides new GC insights d High-resolution rhesus immunoglobulin locus genomic reference sequence

Significantly higher levels of plasma CXCL13 [chemokine (C-X-C motif) ligand 13] were associated with the generation of broadly neutralizing antibodies (bnAbs) against HIV in a large longitudinal cohort of HIV-infected individuals. Germinal centers (GCs) perform the remarkable task of optimizing B-cell Ab responses. GCs are required for almost all B-cell receptor affinity maturation and will be a critical parameter to monitor if HIV bnAbs are to be induced by vaccination. However, lymphoid tissue is rarely available from immunized humans, making the monitoring of GC activity by direct assessment of GC B cells and germinal center CD4 + T follicular helper (GC Tfh) cells problematic. The CXCL13-CXCR5 [chemokine (C-X-C motif) receptor 5] chemokine axis plays a central role in organizing both B-cell follicles and GCs. Because GC Tfh cells can produce CXCL13, we explored the potential use of CXCL13 as a blood biomarker to indicate GC activity. In a series of studies, we found that plasma CXCL13 levels correlated with GC activity in draining lymph nodes of immunized mice, immunized macaques, and HIV-infected humans. Furthermore, plasma CXCL13 levels in immunized humans correlated with the magnitude of Ab responses and the frequency of ICOS + (inducible T-cell costimulator) Tfh-like cells in blood. Together, these findings support the potential use of CXCL13 as a plasma biomarker of GC activity in human vaccine trials and other clinical settings.T he germinal center (GC) reaction is a critical immunological process that occurs in draining lymph nodes after immunization (1). The GC response consists of antigen-specific B cells undergoing affinity maturation through a process of somatic hypermutation (SHM) of the B-cell receptor. SHM is necessary for producing high-affinity Ab responses after immunizations and infections. Influenza neutralizing Abs have substantial SHM. Particularly high levels of SHM, 15-30% amino acid mutation (2, 3), are present and necessary for broad Ab neutralization of diverse HIV strains (4, 5). Therefore, as candidate influenza and HIV vaccines are evaluated for the ability to induce broadly neutralizing antibodies (bnAbs), the quantitation and functional characterization of GC responses will be a key parameter for study. Serological analysis of vaccine-specific Ab titers provides important information, but those data are limited. Serological outcomes are measured at time points long after initial immunizations. Neutralizing Ab responses are commonly only measurable after multiple boosts. Those outcomes likely depend on GC activity and affinity maturation at much earlier time points. Several state of the art HIV vaccine strategies rely on long, multistage immunization protocols (6, 7). With bnAb responses as the goal, means of early analysis of the immune response will be essential to understand and improve on vaccination schemes that may end in failure or only partial success. One critical parameter to assess will be the ability of each immunization to generate GC responses.Central to the GC re...

The identification and study of antigen-specific CD4 T cells, both in peripheral blood and in tissues, is key for a broad range of immunological research, including vaccine responses and infectious diseases. Detection of these cells is hampered by both their rarity and their heterogeneity, in particular with regards to cytokine secretion profiles. These factors prevent the identification of the total pool of antigen-specific CD4 T cells by classical methods. We have developed assays for the highly sensitive detection of such cells by measuring the upregulation of surface activation induced markers (AIM). Here, we compare two such assays based on concurrent expression of CD69 plus CD40L (CD154) or expression of OX40 plus CD25, and we develop additional AIM assays based on OX40 plus PD-L1 or 4-1BB. We compare the relative sensitivity of these assays for detection of vaccine and natural infection-induced CD4 T cell responses and show that these assays identify distinct, but overlapping populations of antigen-specific CD4 T cells, a subpopulation of which can also be detected on the basis of cytokine synthesis. Bystander activation had minimal effect on AIM markers. However, some T regulatory cells upregulate CD25 upon antigen stimulation. We therefore validated AIM assays designed to exclude most T regulatory cells, for both human and non-human primate (NHP, Macaca mulatta) studies. Overall, through head-to-head comparisons and methodological improvements, we show that AIM assays represent a sensitive and valuable method for the detection of antigen-specific CD4 T cells.

Detection of antigen-specific CD4+ T cells is central to the study of many human infectious diseases, vaccines, and autoimmune diseases. However, such cells are generally rare and heterogeneous in their cytokine profiles. Identification of antigen-specific germinal center (GC) T follicular helper (Tfh) cells by cytokine production has been particularly problematic. The function of a GC Tfh cell is to selectively help adjacent GC B cells via cognate interaction; thus, GC Tfh cells may be ‘stingy’ cytokine producers, fundamentally different than Th1 or Th17 cells in the quantities of cytokines produced. Conventional identification of antigen-specific cells by intracellular cytokine staining (ICS) relies on the ability of the CD4+ T cell to generate substantial amounts of cytokine. To address this problem, we have developed a cytokine-independent activation induced marker (AIM) methodology to identify antigen-specific GC Tfh cells in human lymphoid tissue. Whereas Group A Streptococcus (Strep)-specific GC Tfh cells produced minimal detectable cytokines by ICS, the AIM method identified 85-fold more antigen-specific GC Tfh cells. Intriguingly, these GC Tfh cells consistently expressed programmed death ligand 1 (PD-L1) upon activation. AIM also detected non-Tfh cells in lymphoid tissue. As such, we applied AIM for identification of rare antigen-specific CD4+ T cells in human peripheral blood. Dengue-, tuberculosis-, and pertussis-vaccine-specific CD4+ T cells were readily detectable by AIM. In sum, cytokine assays missed 98% of antigen-specific human GC Tfh cells, reflecting the biology of these cells, which could instead be sensitively identified by co-expression of TCR-dependent activation markers.