S epsilon S mu and S epsilon S gamma switch circles in human nasal mucosa following ex vivo allergen challenge: evidence for direct as well as sequential class switch recombination.

B cells switch to IgE under the influence of IL-4, IL-13, and CD40 costimulation through a multistep process involving epsilon germline transcription and class switch recombination. Classically, switching has been considered an event restricted to lymphoid tissues; however, epsilon germline transcripts (I(initiator)epsilon RNA) have been observed within lung, sinus, and nasal tissue of individuals with asthma, sinusitis, and rhinitis. Furthermore, nasal mucosal tissue from allergic rhinitics produces epsilon germline transcripts following ex vivo allergen challenge. Collectively, these studies raised the possibility that switching to IgE may occur locally, at sites of allergic inflammation. Although epsilon germline transcripts are considered necessary to target the IgE locus, it is class switch recombination that ultimately leads to de novo IgE production. In this study, we demonstrate that S epsilon S mu DNA switch circles (products of class switch recombination) as well as I epsilon and C epsilon RNA are produced within nasal tissue from allergic individuals following ex vivo allergen challenge. epsilon germline transcription was inhibited when tissue was cultured with a combination of allergen and neutralizing Abs against IL-4 and IL-13, indicating that de novo cytokine production mediated the isotype switch. We also show allergen-induced appearance of S epsilon S gamma DNA switch circles and up-regulation of C gamma 4 mRNA, illustrating that sequential switching to IgE also occurred. This work strongly suggests that B cells residing within the nasal mucosa undergo switching to IgE in the context of a local immune response to allergen.