BAFF as a Key Modulator of Respiratory Mucosal B Cell Immunity in Viral Infection and Mucosal Vaccination.

Mucosal immunity in the respiratory tract provides the first line of defense against airborne pathogens, yet most current vaccines fail to induce strong and durable immune responses at these sites. Respiratory viruses, including respiratory syncytial virus (RSV), influenza viruses, and coronaviruses, remain major global health threats, in part due to their ability to evade long-term mucosal protection. Although systemic vaccination generates robust circulating immunity, it induces limited local responses, particularly secretory immunoglobulin A (IgA), which is critical for preventing viral entry and transmission at the airway surface. The mechanisms regulating B cell responses within the airway mucosa are not fully understood. B cell-activating factor (BAFF), a member of the tumor necrosis factor (TNF) superfamily, has emerged as an important context-dependent regulator of mucosal B cell immunity. BAFF is produced by airway epithelial cells and multiple myeloid populations, including dendritic cells and neutrophils, and is rapidly induced during respiratory viral infection through type I interferon-dependent pathways. Functionally, BAFF supports B cell survival, differentiation, and class-switch recombination, promoting the generation of antibody-secreting plasma cells and enhancing IgA production. In the lung, these effects align with early, intermediate, and late stages of the response, supporting initial local antibody production, the formation of inducible bronchus-associated lymphoid tissue (iBALT), and the development of tissue-resident memory B cells that sustain long-term immunity. Although BAFF plays an essential role in mucosal immunity, its activity requires tight regulation to maintain immune balance. Current evidence supports BAFF as a promising immunomodulatory component and highlights its potential as an adjuvant platform for enhancing mucosal vaccine efficacy, warranting further investigation as a potential adjuvant in this context.