Evidence for Accessory Cell Function by Class II MHC Antigen-expressing Airway Epithelial Cells

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Abstract
Expression of major histocompatibility complex (MHC) class II antigens is a requirement for accessory cell function in antigen presentation. Recent reports have demonstrated the presence of class II antigens on human bronchial epithelial cells. In the present study, immunohistochemical staining revealed HLADR on human airway epithelial cells obtained from two different mucosal sites (lobar bronchus and nasal turbinates). To determine whether airway epithelial cells bear functional class II molecules that allow for their cognate interaction with T lymphocytes, cells isolated from these sites were used in mixed lymphocyte cultures (MLR), as an in vitro model of accessory cell function. Freshly isolated cells (11 bronchi/3 turbinates) stimulated allogeneic T lymphocytes (stimulation index [S.I.] = 9.3 [mean]; P < 0.001 compared to T cells alone). In order to assess the potential role of contaminating conventional accessory cells, bronchial epithelial cell isolates were first preincubated in a serum-free, growth factor-supplemented medium that functionally eliminates potential non-epithelial stimulators prior to MLR culture. Conventional accessory cell-depleted epithelial cells were still capable of stimulating allogeneic T lymphocytes in 18 of 23 MLR cultures (S.I. = 5.5 [mean]; P < 0.0005 compared to T cells alone). The addition of an anti-class II monoclonal antibody (VG2.2) at the onset of culture completely inhibited the MLR response (n = 10). No shift in the CD4+/CD8+ ratio was detected between lymphocytes harvested from airway epithelial cell MLR (1.42 ± 1.29) and the ratio from T lymphocytes cultured alone (1.3 ± 0.75), suggesting that both CD4+ and CD8+ T lymphocytes were proliferating in response to stimulation from alloepitopes recognized on airway epithelial cells. These studies show that airway epithelial cells can stimulate allorestricted T lymphocytes, and that this capacity appears to be linked to class II antigen expression. This finding supports the hypothesis that airway epithelial cells may perform an accessory cell function, and that presentation of antigen may not be limited to the alveolar or interstitial compartments in which traditional accessory cell populations are found.

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