Dendritic cells within the respiratory mucosa initiate immune responses to foreign and environmental antigens which are continually inhaled into the respiratory tract. We hypothesized that GM-CSF, a pleiotropic cytokine released by respiratory cells in response to allergen, could act on respiratory tract dendritic cells (RTDC) to promote differentiation of Th2 cells.

CD1a+ myeloid DC were purified using magnetic bead technology from nasal turbinates of non-atopic patients with rhinitis. RTDC were immunophenotyped and inflammatory cytokine production in response to LPS and CD40L determined by cytometric bead array (CBA). RTDC were cultured with allogeneic, naïve CD4+ T cells from non-atopic, healthy individuals in the mixed lymphocyte reaction and Th1, Th2 and IL-10 production determined by CBA. T cells were harvested and restimulated with mAb to CD3 and CD28.

RTDC, which expressed an immature phenotype, responded to GM-CSF by upregulation of the maturation marker, CD83, and the co-stimulatory receptor, CD86. Exposure of RTDC to CD40L or LPS significantly increased IL-6 (p<0.05) and IL-8 (p<0.01) production but IL-12p70 production was only detectable at very low levels. In mixed lymphocyte cultures, prior exposure of RTDC to GM-CSF skewed the response of CD4+ T cells from a Th0 to a Th2 profile. Following secondary stimulation, amplification of the Th2 response was observed. CD86 was shown by inhibition experiments to be essential for Th2 cytokine production.

These results directly implicate GM-CSF in amplifying the potential of RTDC to initiate allergic inflammation in the respiratory tract