Protective Vaccine-Induced CD4⁺T Cell-Independent B Cell Responses against Rabies Infection

Abstract

T he prototypical vaccine used in postexposure settings was developed over a century ago for use following exposure to a potentially rabid animal (9, 14). Postexposure vaccination remains the worldwide standard for the prevention of rabies infections of humans. Despite the long history of rabies vaccine use as a postexposure treatment and the facts that over two-thirds of the world population live in regions where rabies is endemic and that over 55,000 people die every year due to RV infections, little information is available regarding the development of effective B cell responses early postvaccination, which may influence the outcome of postexposure vaccine efficacy. Current vaccines for human use are based on inactivated RV strains. Therefore, vaccineinduced protection against RV has long been described as being dependent solely on CD4 ϩ T cell help for the induction of protective antibody responses (9). These observations are supported by experiments carried out in RV-vaccinated nude mice that did not develop anti-RV antibody responses and were not protected against pathogenic challenge in preexposure settings (29). Similarly, Mifune et al. (16) showed that vaccinated athymic nude mice did not produce anti-RV antibodies in postexposure experiments. Furthermore, neutralizing antibodies directed against RV antigens were shown to be produced only with the assistance of Thelper cells (1). Finally, T cell depletion studies further showed the importance of CD4 ϩ T cells in generating neutralizing, protective antibodies in mice after RV infection (21). Together, these experiments showed that current inactivated RV-based vaccines rely on T cell help for the elicitation of effective antibody responses associated with protection against pathogenic challenge.Vaccine-induced immunity is a complex process involving innate and adaptive immune responses. During the development of typical vaccine-induced immunity, antigen-primed T cells migrate to the T and B cell borders of secondary lymphoid organs, where they interact with their cognate antigen-specific B cells. After activation, B cells differentiate into early, short-lived extrafollicular plasma cells, germinal center (GC) B cells, or early unswitched memory B cells that can recirculate (7). Within GCs, B cells differentiate into memory B cells or long-lived plasma cells that secrete high-affinity, postswitched antibodies. In most cases, fully formed GC-derived memory B cells and plasma cells can take from days to weeks to develop. From a traditional vaccination standpoint, this lag time between vaccination and GC B cell development is required and usually acceptable, since GC B cells are critical for long-term B cell responses to protect against future exposure to the pathogen. However, vaccines are also used in postexposure settings after exposure to pathogens, such as for RV. Since the lag time between vaccination and the generation of effective GC B cell responses might be too long, administration of postexposure vaccines for the prevention of rabies infection may...

B Cell Infection and Activation by Rabies Virus-Based Vaccines

“…The localized T cell activation may then in turn quickly provide help to RABV-specific B cells, driving them toward early IgG classswitched antibody production. This sequence of events is consistent with our previous data showing the rapid induction of T cell-dependent IgG isotype antibodies within 5 days of immunization and could contribute to the efficacy of a live, single-dose RABV-based vaccine (10,13). Possible T-independent effects on the RABV-specific antibody response, such as the stimulation of antibody secretion or class-switching in RABV-infected B cells, were not examined in the current study but remain to be elucidated, as T-independent antibody production is an important mechanism contributing to the rapid antibody responses needed in postexposure settings (13).…”

“…Previously we described the early, protective, extrafollicular antibody response to immunization with replication-deficient rRABV-⌬M as containing both T-independent and T-dependent antibody production components (13). Our discovery that B cells infected with rRABV are activated to a costimulatory antigenpresenting phenotype suggests a novel means of initiation and support of the early T-dependent component of this antibody response.…”

“…Infection of B cells by rRABV carries a number of implications for RABV-based vaccine therapy. Previously, we observed prompt and potent B cell activation and antibody secretion by rRABV immunization (13). We hypothesized that direct infection of B cells with rRABV results in early B cell activation, providing additional stimulus to both T-independent and T-dependent antibody responses.…”

“…In an attempt to maintain the B cells and accessory splenocytes in a resting state similar to that in which they would exist in vivo at the time of initial immunization, no additional mitogens were added to the culture. Figure 1A shows a representative gating strategy of total live splenocytes stained for intracellular RABV nucleoprotein (RABV-N) as a marker of infection and the B cell marker B220 (CD45R) (13,19). By 4 days postinfection, approximately 60% of the splenocyte cultures were B220 ϩ B cells, 10% were CD4 ϩ T cells, and 30% were non-B/non-T cells.…”

“…A particular feature of the antibody response to rRABV-⌬M is the presence of VNAs before B cells displaying a germinal center (GC) phenotype are detected, suggesting the induction of early extrafollicular antibody responses by rRABV-⌬M (13). Indeed, contrary to earlier reports citing the necessity of CD4 ϩ T cells for protective RABV-specific B cell responses (14)(15)(16)(17)(18), we detected the presence of significant VNA titers within 3 days postimmunization with rRABV-⌬M and protection against lethal challenge in mice completely devoid of T cells (B6.129P2-Tcr␤ tm1Mom Tcr␦ tm1Mom /J) (13). Based on these data, we concluded that both early T cell-dependent and independent antibody responses contribute to the early protection observed with rRABV-⌬M.…”