Regulation of CD74 expression in response to Human Interferon Gamma and Lipopolysaccharide on Human Trophoblast Derived Cells: Relevance for Human Feto-Maternal Tolerance.

During pregnancy, the maternal immune system protects the allogeneic foetus from rejection. At the same time the mother maintains immunity defences against potential pathogens. This study aimed to identify whether immunological receptors associated with antigen presentation and strong inflammatory responses play a role in fetal-maternal tolerance. One such receptor is CD74; a membrane-bound protein involved in HLA class II- mediated antigen presentation and a macrophage migration inhibitory factor (MIF) receptor. CD44 is the signalling component of the MIF-CD74 receptor complex. The expression of CD74, MIF and CD44 was studied in the human trophoblast-derived cell lines JEG-3 and ACH-3P by RT-PCR, flow cytometry, Western blot, immunoprecipitation and fluorescence microscopy. Results obtained showed that untreated JEG-3 and ACH-3P cells did not express CD74 mRNA. By contrast, CD74 mRNA was upregulated in response to IFN-γ or LPS in these cell lineages. Slight upregulation of CD74 was observed following exposure of cells to 500 IU/ml IFN-γ for 12 hr. However, after 5 µg/ml LPS treatment for 4 hr, CD74 was highly upregulated. Results from flow cytometry showed no detectable surface expression of CD74 in the JEG-3 and ACH-3P cell lines even after IFN-γ or LPS treatment. However, IFN-γ and LPS exposure resulted in intracellular expression of CD74 in both cell lines. Western blotting showed the absence of a protein band for CD74 in both cell lines after IFN-γ treatment. However, the 35 kDa isoform of CD74 was detected in JEG-3 and ACH-3P cells treated with LPS. The results of this study indicate that LPS regulates the expression of MIF and CD44 and trophoblast cell proliferation. It was also demonstrated that CD74 positivity significantly increased after incubation with IFN-γ or LPS, and that MIF and CD44 are up-regulated by LPS. No evidence was found for colocalization between CD74 with either CD44 or MIF in JEG-3 and ACH-3P cells using confocal microscopy and coimmunoprecipitation. This indicated that MIF plays an essential role in cell proliferation, whereas both MIF and CD44 play an important role in human pregnancy maintenance. Together, the results suggest that the absence of cell surface expression specific isoforms of CD74 may provide a protective role by minimising inflammatory processes, and thus maximising a healthy pregnancy. In turn, it is reasonable to assume that the overexpression of CD74 in early pregnancy may be related to gestational complications.