Humoral Immune Response to Flagellin Requires T Cells and Activation of Innate Immunity

Abstract

Bacterial flagellin, the primary structural component of flagella, is a dominant target of humoral immunity upon infection by enteric pathogens and in Crohn’s disease. To better understand how such responses may be regulated, we sought to define, in mice, basic mechanisms that regulate generation of flagellin-specific Igs. We observed that, in response to i.p. injection with flagellin, generation of flagellin-specific Ig required activation of innate immunity in that these responses were ablated in MyD88-deficient mice and that flagellin from Helicobacter pylori, which is known not to activate TLR5, also did not elicit Abs. Mice lacking αβ T cells (TCRβnull) were completely deficient in their ability to make flagellin Abs in various contexts indicating that, in contrast to common belief, generation of flagellin-specific Ig is absolutely T cell dependent. In contrast to Ab responses to whole flagella (H serotyping), responses to flagellin monomers displayed only moderate serospecificity. Whereas neither oral nor rectal administration of flagellin elicited a strong serum Ab response, induction of colitis with dextran sodium sulfate resulted in a MyD88-dependent serum Ab response to endogenous flagellin, suggesting that, in an inflammatory milieu, TLR signaling promotes acquisition of Abs to intestinal flagellin. Thus, acquisition of a humoral immune response to flagellin requires activation of innate immunity, is T cell dependent, and can originate from flagellin in the intestinal tract in inflammatory conditions in the intestine.

Anti-flagellin antibody responses elicited in mice orally immunized with attenuated Salmonella enterica serovar Typhimurium vaccine strains

“…Thus, the generation of flagellin preparations with higher purity and the use of a nonflagellated strain helped us to demonstrate that mice p.o. with flagellated S. Typhimurium strains failed to mount significant systemic and secreted flagellin-specific antibody responses, similar to results previously reported for S. Dublin strains (Sbrogio & Almeida 2001, Sbrogio et al 2004, Sanders et al 2006.…”

“…Interestingly, the genetic background of both the mouse lineage and the Salmonella strain affected the immunogenicity of flagellins delivered orally using recombinant vaccine strains (Sbrogio-Almeida 2004). Indeed, recent evidence indicates that Salmonella flagellin administered via the oral route may trigger immunological hyporesponsive responses in healthy mice, although the precise mechanism underlying these responses remains unknown (Sanders et al 2006).…”

Deletion of Flagellin’s Hypervariable Region Abrogates Antibody-Mediated Neutralization and Systemic Activation of TLR5-Dependent Immunity

“…The deletion of major Th or B epitopes may explain the decreased ability of FliC ⌬174 -400 to elicit Abs. Previous studies identified a dominant Th epitope in flagellin's conserved domain and highlighted a major role for CD4 T cells in Ab responses (29,36). Deletion of flagellin's hypervariable region is therefore not absolutely required for effective help for B cell responses.…”

The role of T‐regulatory cells and Toll‐like receptors in the pathogenesis of human inflammatory bowel disease

“…84 Remarkably, even in wild-type mice, chemically induced colitis is associated with the development of anti-flagellin immunoglobulin responses, indicating that flagellin can be a dominant antigen in both spontaneous and experimentally induced colitis. 85,86 Immune responses to flagellin are also implicated in human IBD: 84,[87][88][89] subsets of patients with CD, but not with UC, have elevated antibody responses to CBir flagellin, 84,88 as well as to a wider variety of flagellin subtypes. 87,88 That flagellin is the only known human TLR ligand which is a protein, and can thus activate both the innate and adaptive immune systems in parallel, may be one reason why this protein is a dominant antigen in IBD.…”