No abstract

No abstract

Human respiratory syncytial virus (RSV) is a serious respiratory pathogen in infants and

Naive peripheral B cells are maintained in sufficient numbers and diversity to mount effective immune responses against infectious agents. However, the size and repertoire of this B cell pool is constantly diminished by normal cell turnover and Ag activation. Homeostatic (Ag-independent) proliferation in response to B cell depletion is one mechanism to compensate for this cell loss. We have used purified CFSE-labeled B cells and an adoptive transfer model system to show that immature and mature B cells divide in a variety of B cell-deficient (scid, xid, IL-7−/−, and sublethally irradiated) hosts. Homeostatic B cell proliferation is T cell independent, and B cells that have replicated by this mechanism retain the antigenic phenotype of naive B cells. Replication is significantly reduced in B cell-sufficient normal or B cell-reconstituted immunodeficient recipients by the action of competing mature follicular B cells. Using xid mice and transcription factor knockouts, we show that the activation signal(s) that lead to homeostatic B cell proliferation require Bruton’s tyrosine kinase; however, c-Rel, a Bruton’s tyrosine kinase-induced NF-κB/Rel transcription factor critical for Ag and mitogen stimulation, is dispensable, indicating the uniqueness of this activation pathway. Survival and replication signals can also be separated, because the transcription factor p50 (NF-κB1), which is required for the survival of peripheral B cells, is not necessary for homeostatic replication. Homeostatic B cell proliferation provides an Ag-independent mechanism for the maintenance and expansion of naive B cells selected into the mature B cell pool.

Virus IMPORTANCEThe development of vaccines for respiratory syncytial virus has been hampered by a lack of understanding of the requirements for eliciting high titers of neutralizing antibodies. The results of this study suggest that particle-associated RSV F protein containing mutations that stabilize the structure in a prefusion conformation may stimulate higher titers of protective antibodies than particles containing F protein in a wild-type or postfusion conformation. These findings indicate that the prefusion F protein assembled into VLPs has the potential to produce a successful RSV vaccine candidate.H uman respiratory syncytial virus (RSV) is the most significant cause of acute viral respiratory disease in infants and young children (1). There are from 34 to 65 million RSV infections resulting in acute lower respiratory disease requiring hospitalization and 160,000 to 199,000 deaths per year worldwide (2). Elderly populations are also at significant risk for serious RSV disease. In the United States, the virus accounts for 10,000 deaths and 14,000 to 60,000 hospitalizations per year among individuals more than 64 years of age (3-5). Indeed, RSV infection of this population is at least as significant as influenza virus infections. RSV infections result in high mortality rates in immunocompromised populations, particularly stem cell transplant recipients (6) and individuals with cardiopulmonary diseases (7). Despite the significance of RSV disease in different populations, there are no vaccines available.Failure to develop a licensed RSV vaccine is not due to lack of effort as numerous vaccine candidates have been characterized in preclinical and clinical studies spanning 5 decades (summarized in references 8 to 9). While many problems have uniquely hindered RSV vaccine development, a major hurdle has been a lack of understanding of requirements for generation of protective immunity to RSV infection. Many vaccine candidates are protective in animal models and, while stimulating antibody responses in humans, have failed to induce high levels of neutralizing antibodies and protection from virus challenge in human trials (reviewed in references 10 and 11). Although there are likely many reasons for these observations, one important issue has been a lack of clear understanding of the most effective form of the RSV antigens, particularly the F protein, for stimulating potent neutralizing antibodies.The paramyxovirus F protein is folded into a metastable conformation and upon fusion activation refolds, through a series of conformational intermediates, into the postfusion conformation, which is structurally very different from the prefusion form (12)(13)(14)(15)(16)(17)(18)(19). While it is logical to assume that the prefusion form of F protein should be more effective in stimulating optimally neutralizing antibodies, recent structural studies have shown that the postfusion form of the F protein contains at least some epitopes recognized by neutralizing monoclonal antibodies (17, 18). Thus, it has been a...