Products of the COX enzymes, particularly PGE2
, significantly affect immune cell function and participate in the pathogenesis of several autoimmune diseases (36
). The discovery of COX-2, which is induced by inflammatory stimuli, led to the belief that the constitutive isoform, COX-1, had little to no involvement in regulating the immune response (7
). However, recent studies suggest that COX-1 may be actively involved in immunoregulation (36
). In this report, we demonstrate that COX-1 products fulfill an unsuspected critical role in the humoral immune response by promoting isotype switching and the efficient development of pathogen-specific IgG via IL-17 production.
Increasing evidence indicates that IL-17 plays a significant role in antibody production. IL-17−/−
mice developed lower levels of anti-TNP antibodies in a model of contact hypersensitivity (51
), and GC development and isotype-switching were impaired in a model of autoimmune arthritis (3
). Studies investigating the role of PGE2
in the regulation of immunity with regard to IL-17 have been conducted primarily in vitro
and have utilized exogenous PGE2
. Incubation of naïve human or mouse T cells with PGE2
concomitant with activation led to an increased number of cells differentiated to the T helper (TH
) 17 phenotype with enhanced IL-17 production per cell (52
). Because PGE2
production is responsive to IL-17, and vice versa, it is tempting to conclude that as the “pro-inflammatory” COX isoform, COX-2 is necessary for governing this response. However, experiments in which PGE2
production was inhibited in the context of T cell IL-17 production have utilized indomethacin, a non-specific inhibitor of COX activity, leaving open the question of which COX isoform was in fact responsible for modulating IL-17 production.
To date, no studies have examined the differential activity of COX-1 and COX-2 in B cells. Our data demonstrated that normal murine B cells express both COX-1 and COX-2, and are capable of producing PGE2
, as well as PGF2α
via either enzyme. B cells have the capacity to respond to these eicosanoids in an autocrine manner, since both FP and TP were expressed upon stimulation. These data build upon earlier work, where PGF2α
was demonstrated to affect DNA synthesis and excision repair in murine splenocytes (54
), implying that this lipid may be essential for immunoglobulin isotype switching. Blockade of FP resulted in a significant decrease in IgM and IgG in vitro,
equivalent to the suppression seen using the COX-1-specific inhibitor SC-560, demonstrating that products of COX-1 other than PGE2
, such as PGF2α
may influence the immune response by a number of pathways, one of which could be autocrine regulation of Ig production. Although we have demonstrated that murine B cells produce PGF2α
and TXB2 in vitro
, further studies are needed to determine which cells of the splenic microenvironment produce these eicosanoids in vivo
and whether other cells of the germinal center, such as follicular dendritic cells or T cells, are regulated by these same eicosanoids in an autocrine manner, subsequently regulating IL-6 and IL-17 production.
Neither COX-1 nor COX-2 have been linked to GC formation; however, COX-2 has been mentioned tangentially as a contributor to humoral immunity, with the assumption that the effect was mediated via alteration of the TH
2 axis (29
). Our data demonstrate that animals deficient in COX-1 demonstrated abnormal splenic architecture and failed to develop normal GC following infection with B. burgdorferi
. Additionally, although COX-1 activity was necessary for the production of normal serum IL-6 and IL-17 levels, the addition of exogenous IL-17 alone fully restored pathogen-specific IgG production and borreliacidal activity in mice treated with a COX-1-specific inhibitor, supporting the hypothesis that IL-17 functions in the normal humoral immune response to infection. Furthermore, although previous studies have demonstrated that IL-17 can increase COX-2 mRNA production and, conversely, that the COX product PGE2
can increase IL-17 production, this is the first study to identify COX-1 as the isozyme that influences IL-17 in vivo
. Additionally, although studies have focused on the contribution of PGE2
as the master eicosanoid involved in modulation of the acquired immune response, lipidomic analyses revealed copious production of several other COX products in the spleen, chiefly PGF2a
, illustrating the unexplored potential for other in vivo
lipid mediators of IL-17-mediated antibody production.
Since dual inhibition in vivo had the earliest effect on pathogen-specific IgG production, this implies that compensation may be a factor when inhibition of only one COX isozyme is employed. This also indicates that drugs that inhibit both isozymes equally (tNSAIDs) may have a greater clinical effect with regard to antibody production by blocking compensatory activity of COX-2 during COX-1 inhibition. The use of COX-1° mice clearly illustrated the pivotal role of the COX-1 isozyme in the development of pathogen-specific antibody responses, as these animals demonstrated increased IgM and decreased B. burgdorferi-specific IgG, indicative of a class-switching defect.
Both IL-6 and IL-17 are mediators of the adaptive immune response to Borrelia
infection. IL-6° mice infected with Borrelia burgdorferi, Listeria monocytogenes,
or vesicular stomatitis virus, among others, have significantly lowered serum levels of antigen-specific IgG (55
). More recently the contribution of IL-17 as a modulator of the humoral immune response has been explored in models of autoimmunity. Decreased levels of IL-17 correlated with decreased autoantibody production in several animal models of autoimmune disease (57
), which has subsequently been linked to the regulation of GC generation (3
). Although these previous studies indicated a role for IL-17 in the genesis of autoantibodies, the current study is the first to demonstrate that IL-17 plays a role in the generation of antibodies during the normal humoral immune response to infection. The connection between COX-1, IL-17, and humoral immunity was confirmed in the borreliacidal activity assay. The ability of antibodies to kill Borrelia
is commonly used as a diagnostic tool to determine the efficacy of clinical treatments for Lyme disease (34
). We therefore utilized this assay to demonstrate that the decreased antibody levels induced by COX-1 inhibition lead to a clear functional decrease in the ability of antibodies to kill Borrelia
spirochetes. This defect was recovered in animals treated with exogenous IL-17, restoring borrreliacidal activity in animals treated with COX-1 inhibitors and establishing a clear role for COX-1 activity in the generation of a functional humoral immune response.
The use of the B. burgdorferi
infection model demonstrates that in response to a clinically-relevant pathogen, products of COX-1 govern the humoral response via regulation of IL-6 and IL-17 production, and the generation of GC. The description of this previously unappreciated role for COX-1 not only expands our understanding of the regulation of humoral immunity, but comes at a time when the use of alternatives to COX-2-specific inhibitors is on the rise (60
) in addition to the already prevalent use of tNSAIDs (19
). Although it is predicted that patients using COX-1-specific inhibitors or tNSAIDs may be more susceptible to infectious agents or have decreased responses to vaccine preparations, this study also reveals COX-1 as a potential therapeutic target in conditions caused by pathologic antibody production.