The nasal mucosa, an important arm of the mucosal immune system, is the first site of contact with inhaled antigens to induce an IgA response. A major aim of this study was to characterize the Th1 ...and Th2 cytokine expression of mucosal T cells residing in nasal‐associated lymphoid tissue (NALT) and nasal passages (NP) as IgA inductive and effector sites, respectively, at the transcription and cellular levels. An application of single‐cell reverse transcription‐PCR for analysis of Th1 (IFN‐γ) and Th2 (IL‐4 and IL‐6) cytokine‐specific mRNA revealed the presence of CD4+ T cells with a Th0 profile in NALT, while high numbers of Th2 cytokine‐specific mRNA expressed by CD4+ T cells were noted in NP followed by Th1‐type cells. NALT CD3+ CD4+ T cells of Th0 type have the capacity to become Th1‐ and/or Th2‐type cells since their activation via the TCR‐CD3 complex resulted in the expression of an array of Th1 and Th2 cytokines. CD3+ CD4+ T cells from NP, but not NALT, provide a helper function for the induction of antibody‐forming cells including IgA isotype in B cell cultures. These findings suggest that NALT is characterized by a Th0 environment which can gain a Th1 and/or Th2 phenotype. In contrast, NP is considered to be a Th2 dominant site with some Th1 cells that can support the induction of IgA‐producing cells.
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