Myeloid dendritic cells (mDCs) have long been thought to function as classical APCs for T cell responses. However, we demonstrate that influenza viruses induce rapid differentiation of human monocytes into mDCs. Unlike the classic mDCs, the virus-induced mDCs failed to upregulate DC maturation markers and were unable to induce allogeneic lymphoproliferation. Virus-induced mDCs secreted little, if any, proinflammatory cytokines; however, they secreted a substantial amount of chemoattractants for monocytes (MCP-1 and IP-10). Interestingly, the differentiated mDCs secreted type I IFN and upregulated the expression of IFN-stimulated genes (tetherin, IFITM3, and viperin), as well as cytosolic viral RNA sensors (RIG-I and MDA5). Additionally, culture supernatants from virus-induced mDCs suppressed the replication of virus in vitro. Furthermore, depletion of monocytes in a mouse model of influenza infection caused significant reduction of lung mDC numbers, as well as type I IFN production in the lung. Consequently, increased lung virus titer and higher mortality were observed. Taken together, our results demonstrate that the host responds to influenza virus infection by initiating rapid differentiation of circulating monocytes into IFN-producing mDCs, which contribute to innate antiviral immune responses.

Background. Influenza disproportionately impacts older adults while current vaccines have reduced effectiveness in the older population.Methods. We conducted a comprehensive evaluation of cellular and humoral immune responses of adults aged 50 years and older to the 2008–2009 seasonal trivalent inactivated influenza vaccine and assessed factors influencing vaccine response.Results. Vaccination increased hemagglutination inhibition and neutralizing antibody; however, 66.3% of subjects did not reach hemagglutination inhibition titers ≥ 40 for H1N1, compared with 22.5% for H3N2. Increasing age had a minor negative impact on antibody responses, whereas prevaccination titers were the best predictors of postvaccination antibody levels. Preexisting memory B cells declined with age, especially for H3N2. However, older adults still demonstrated a significant increase in antigen-specific IgG+ and IgA+ memory B cells postvaccination. Despite reduced frequency of preexisting memory B cells associated with advanced age, fold-rise in memory B cell frequency in subjects 60+ was comparable to subjects age 50–59.Conclusions. Older adults mounted statistically significant humoral and cell-mediated immune responses, but many failed to reach hemagglutination inhibition titers ≥40, especially for H1N1. Although age had a modest negative effect on vaccine responses, prevaccination titers were the best predictor of postvaccination antibody levels, irrespective of age.

Aging is associated with a decline in immune function (immunosenescence) that leads to progressive deterioration in both innate and adaptive immune functions. These changes contribute to the subsequent increased risk for infectious diseases and their sequelae. Vaccination is the most effective and inexpensive public health strategy for prevention of infection, despite the decreased efficacy of vaccines in older adults due to immunosenescence. The rapid rise in the older adult population globally represents a great challenge for vaccination programs. This article first addresses the status of innate and adaptive immune functions in aging and then focuses on influenza vaccine. The development history of influenza vaccines, current status, and potential strategies to improve the immunogenicity and vaccine effectiveness in older adults are discussed.

Influenza virus-specific memory B cells are critical to mount an accelerated antibody response against subsequent influenza infections. While multiple layers of humoral memory are involved in the protection against influenza, the unique role of IgM memory B cells is largely unknown. In this study, we demonstrated that human IgM memory B cells are similar to class-switched memory B cells in their basic effector functions: the ability to proliferate and to present antigens to T cells. However, ELISPOT analysis of peripheral blood mononuclear cells revealed that the frequency of influenza-specific IgM memory B cells is 10-fold higher than that of IgG memory B cells and doesn’t readily change by seasonal vaccination. Interestingly, IgM memory B cells exhibited broad cross-reactivity against contemporary seasonal as well as pandemic strains. Serum antibody analysis showed that although the level of influenza-specific IgM was approximately 20-fold less than that of IgG, the breadth of cross-reactivity of IgM-secreting cells is broader than that of IgG counterparts. Given that a recent study in mice showed that the majority of IgM memory B cells are preferentially recruited to the ensuing secondary response, we propose a model that upon vaccination, some IgM memory B cells are directly differentiated into highly cross-reactive IgM-secreting plasmablasts to confer the host early line of cross-protection, while the rest of IgM memory B cells are recruited to the secondary response.