Here we identify a critical role for TACI and its ligands BLyS and APRIL in the maintenance of protective antiviral ASCs. Following infection with influenza, ASCs increase at various anatomical sites, including the RT, where they are sustained at elevated numbers for several months (Figure C and Figure C). The majority of lung ASCs exhibit low cell cycle activity, suggesting that they are long-lived (Supplemental Figure 2C). However, unlike long-lived ASCs resident in the spleen and BM, ASCs in the lungs of influenza-infected mice are susceptible to the effects of whole-body irradiation (Figure , B and C), which points toward a cell-extrinsic mechanism for ASC survival.
We demonstrate that neutralization of BLyS and APRIL, but not BLyS alone, 5 weeks after infection results in substantially decreased virus-specific ASCs in BM and lungs, correlating with reduced Ab titers in serum and BAL (Figure and Supplemental Figure 3). Interestingly, April–/–
mice or April
-transgenic mice showed normal early antiviral ASC and Ab responses (65
); however, the persistence of ASCs was not investigated in these studies. Here we demonstrate that the early antiviral Ab and ASC response (day 8 p.i.) was similar in WT and TACI–/–
mice (Figure , A and B, and Figure A), but by 3–4 weeks p.i. TACI-deficient mice exhibited significantly lower antiviral Ab titers in serum and BAL correlating with reduced numbers of virus-specific ASCs (Figure , A and B, and Figure B). In contrast to TACI–/–
mice, there was no difference in the humoral response to influenza virus in BCMA–/–
mice (Supplemental Figure 7). Although BCMA has been implicated in survival of BM ASCs (25
), our data and those of others (66
) suggest that BCMA does not play a major role in mediating survival of virally induced ASCs.
In vitro studies have indicated that signals through TACI can play an important role in the induction of class switch recombination (41
). Interestingly, our data suggest that the initial induction of isotype switching after influenza virus infection is largely TACI independent in vivo: The number of virus-specific IgG- and IgA-producing cells in medLN of WT and TACI–/–
mice was similar at 6–8 days p.i. (Figure A). We cannot exclude the possibility that TACI functions in ASC class switching at later times, particularly in the lung, when costimulatory signals and help from effector T cells may have become sparse. However, we demonstrate that blockade of CD40L 5 weeks p.i. does not impact antiviral ASCs and Ab production locally in the lungs (Figure , B–D).
In humans, individuals carrying mutations in the TNFRSF13B
gene encoding TACI are prone to developing CVID, characterized by low serum IgG and IgA concentrations and recurrent bacterial infections (67
). TACI mutations also result in severely decreased proportions of isotype-switched B cells, suggesting defects in the generation and/or maintenance of memory B cells in the absence of functional TACI (68
). TACI mutations can interfere with class switch recombination in humans, potentially through a failure to properly induce activation-induced deaminase (AID
) expression (43
). In addition, naive B cells from TACI-mutated individuals fail to secrete IgA Abs after stimulation in vitro with BLyS or APRIL, suggesting an important role for TACI in human mucosal immunity (67
). In line with this observation, CVID patients often experience pneumonia, potentially leading to chronic lung disease (70
). Moreover, defects in the expression of TACI, which we showed in the present study to be important for the survival of long-lived ASCs in the BM, may also account for the decreased serum IgG and IgA concentrations observed in patients with TACI mutations (67
Strikingly, we show a site-specific requirement for TACI in the antiviral ASC response. Lung and BM ASCs, but not ASCs in medLN and spleen, were significantly reduced after treatment with TACI-Fc or in TACI–/–
mice 5 weeks p.i. (Figure , B and C, and Figure B). Our findings raise the question as to what could account for the difference in TACI-dependent survival of virus-specific ASCs at these sites. Organ-specific access to TLR ligands, for example at mucosal sites, could positively regulate BLyS and/or APRIL expression (26
) and thus influence local isotype switching and/or ASC survival. Studies with human B cells have suggested that mucosal IgG and IgA responses to poly(I:C), a surrogate for viral dsRNA and a ligand for TLR3, can be potentiated with BLyS (27
). In addition to the more ubiquitously present myeloid cell types, specialized cell subsets in the RT such as airway epithelial cells and alveolar macrophages could allow for local regulation of APRIL and BlyS production (26
). Geurtsvankessel at al. recently demonstrated that local depletion of CD11c+
DCs in the lungs of influenza-infected mice coincided with decreased IgA titers in airway secretions and ASCs in the lung (73
). This treatment could have removed a source of cells that are able to produce ASC survival factors. Indeed, we show in this study that CD11b+
cells (including DCs and alveolar macrophages) isolated from lungs exhibited high levels of April
mRNA (Figure D). Moreover, we demonstrate that CD11b+
cells included subsets with increased BLyS and APRIL expression as compared with cells that were CD11b–
. Importantly, we show that these CD11b+
cells were reduced in TACI–/–
mice (Figure , A and B, Supplemental Figure 8A), suggesting a model in which TACI signaling in innate cell subsets indirectly regulates ASC differentiation and survival. Of note, whole body irradiation used in this study to determine the lifespan of ASCs may have ablated these cells, thus indirectly affecting ASC survival in the RT (Figure , B and C). Moreover, it has been demonstrated that TACI may be involved in human monocyte maturation and differentiation (39
). A reduction in macrophages was also observed in APRIL-deficient mice infected with influenza virus (65
). Our results support the possibility, therefore, that the predisposition to lung infections in TACI-deficient individuals is mediated through a failure to maintain isotype-switched ASCs in the lungs by a mechanism involving CD11b+
cells. We are currently investigating CD11b+
cell subsets in lungs (Supplemental Figure 8B) as well as lymphoid organs during influenza infection, with particular attention to eosinophils (74
) and basophils (75
), which have been implicated in controlling ASC survival.
Finally, we demonstrate that reduction of virus-specific ASCs in the lungs and BM correlates with significantly lower virus-specific Ab titers in the airways and serum. The importance of antiviral Abs for protection against reinfection is well documented (1
). There are several mechanisms for antiviral Abs to control influenza infection; the most efficient is through viral neutralization mediated predominantly by Abs against hemagglutinin (77
). We show that decreased serum Ab-titers in TACI–/–
mice translated into a significant reduction in HI (Figure A) and less efficient cross-reactive binding of serum Abs to another H1N1 virus (Figure B). Importantly, this abrogates the protection of mice against secondary viral infection with an antigenic variant virus (Figure D), such as those that arise during antigenic drift of influenza strains.
Taken together, our observations reveal a critical role for TACI in generating optimal humoral immunity and maintaining protection against secondary viral infection. Targeting TACI on both B cells and innate cells for enhanced antiviral ASC survival could lead to improved Ab titer maintenance and protection and may inform future vaccine strategies.