Surfactant protein (SP)-D suppresses antigenic and mitogenic T cell activation in vitro

Abstract

Rationale Pulmonary SP-D is an important immunomodulator but its mechanism of action remains unknown. To study the hypothesis that SP-D exerts its effects directly on T cells, we established an in vitro antigen rechallenge model using Aspergillus fumigatus (Af)-specific lymphocytes. Methods Lymphocytes isolated from spleens and the thoracic lymphnodes of mice sensitized and challenged with Af were studied by tritiated thymidine uptake and by FACS analysis of CFSE expression in vitro. Cytokine release was investigated by ELISA. Recombinant SP-D was purified from the supernatant of an SP-D producing CHO cell line and administered to cells at physiologically relevant 0.1–1.0 ug/ml concentrations. Results SP-D inhibited thymidine incorporation, IL-4 and IL-5 production in response to Af stimulation in a dose-dependent manner. FACS analysis of CFSE distribution showed that the proportion of Af-stimulated CD4 (but not CD8) cells was increased in the M1/2 and decreased in the M3/4 mitotic generations by SP-D treatment indicating a specific inhibitory effect on antigen-driven Th cell proliferation. Further, SP-D inhibited PMA and ionomycin-stimulated T cells and cells induced by PHA in both their proliferative and cytokine responses. The effects of PHA was partially abolished when IL-2 (10ng/ml) was added to the culture or when addition of SP-D was delayed by 24h suggesting that SP-D acts at the early phases of T cell activation. Conclusions SP-D may be an essential immunoregulator in the lung. The regulatory effects of this molecule are mediated by direct inhibition of CD4+ T cell function at the initial phase of their activation.