Plasma cells (PCs) derived from germinal centers (GCs) secrete the high-affinity antibodies required for long-term serological immunity. Nevertheless, the process whereby GC B cells differentiate into PCs is uncharacterized, and the mechanism underlying the selective PC differentiation of only high-affinity GC B cells remains unknown. In this study, we show that differentiation into PCs is induced among a discrete subset of high-affinity B cells residing within the light zone of the GC. Initiation of differentiation required signals delivered upon engagement with intact antigen. Signals delivered by T follicular helper cells were not required to initiate differentiation but were essential to complete the differentiation process and drive migration of maturing PCs through the dark zone and out of the GC. This bipartite or two-signal mechanism has likely evolved to both sustain protective immunity and avoid autoantibody production.

Broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoproteins (Envs) have proven difficult to elicit by immunization. Therefore, to identify effective Env neutralization targets, efforts are underway to define the specificities of bNAbs in chronically infected individuals. For a prophylactic vaccine, it is equally important to define the immunogenic properties of the heavily glycosylated Env in healthy primates devoid of confounding HIV-induced pathogenic factors. We used rhesus macaques to investigate the magnitude and kinetics of B cell responses stimulated by Env trimers in adjuvant. Robust Env-specific memory B cell responses and high titers of circulating antibodies developed after trimer inoculation. Subsequent immunizations resulted in significant expansion of Env-specific IgG-producing plasma cell populations and circulating Abs that displayed increasing avidity and neutralization capacity. The neutralizing activity elicited with the regimen used was, in most aspects, superior to that elicited by a regimen based on monomeric Env immunization in humans. Despite the potency and breadth of the trimer-elicited response, protection against heterologous rectal simian-HIV (SHIV) challenge was modest, illustrating the challenge of eliciting sufficient titers of cross-reactive protective NAbs in mucosal sites. These data provide important information for the design and evaluation of vaccines aimed at stimulating protective HIV-1 immune responses in humans.

Vaccine-elicited mAbs bind the HIV-1 Env CD4bs differently from broadly neutralizing infection-induced mAbs.

Significance: The development of broadly neutralizing antibodies (bNAbs) to HIV-1 is often thought to be a key component of a successful vaccine. A common target of bNAbs is the conserved CD4 binding site (CD4bs) on the HIV envelope glycoprotein (Env) trimeric spike. Although CD4bs-directed bNAbs have been isolated from infected individuals, elicitation of such bNAbs by Env vaccination has proven difficult. To help understand the limitations of current immunogens, we structurally characterized two vaccine-elicited, CD4bs-directed non-bNAbs from primates. We demonstrate that these vaccine-elicited Abs attempt a vertical approach to the CD4bs, thereby clashing with the variable region of the trimeric spike cap, whereas CD4bs-directed bNAbs adopt angles of approach that avoid such clashes. This analysis can inform future vaccine redesign.