Properties of IgA-Binding Receptors on Murine T Cells: Relative Importance of FcαR, β-Galactosyltransferase and Anti-Secretory Component Reactive Proteins (ASCP)

ABSTRACT

Murine T cells and T-cell lines express receptors for the Fc of IgA (FcαR); however, their molecular properties remain to be elucidated. In the present study, we examined three candidate molecules for IgA-binding receptors including FcαR, β-galactosyltransferase (β-GT) and anti-secretory component (SC) reactive proteins (ASCP) expressed on T cells which might participate in the binding of different molecular forms of IgA. T-cell lines derived from CD4⁺ T cells of mouse Peyer's patches (PP) (designated PPT 4–6and PPT 4–16) and from cloned PP T helper (Th) cell lines (ThHA₁#9and #10) bound both monomeric and dimeric IgA (mIgA and dIgA), while the fusion partners (BW 5147 and R 1.1) did not. In contrast, both FcαR⁺ and FcαR^- cell lines bound to high molecular weight polymeric or aggregated IgA (pIgA). All cell lines reacted with a monoclonal anti-β-GT(MoAb) and β-GT enzyme activity was associated with the cell lysates and membrane fractions of all cells tested. The anti-β-GT MoAb stained a 47-kDa band on immunoblots which was identical to that seen with native enzyme. mRNA analysis with β-GT cDNA showed that all cell lines constitutively produced enzyme-specific mRNA. Both FcαR⁺ T cells and FcαR^- control cell lines showed cell surface specific β-GT activity. This is the first study which shows that mouse T cells produce β-GT. However, FcαR and β-GT appear to be separate receptors, because FcαR⁺ T cells bound mIgA and dIgA, and this treatment did not affect staining with biotinylated anti-β-GT MoAb. Further, preincubation of the FcαR⁺ cells with anti-β-GT MoAb did not block mIgA binding. However, the anti-β-GT MoAb partially blocked binding of pIgA to both FcαR⁺ and FcαR^- T cells, suggesting that β-GT may be a receptor for pIgA. Others have shown that T cells may bind IgA through a receptor serologically related to SC. We found that antibodies both to human SC and to rat SC specifically bound to both FcαR⁺ and FcαR^- T cells. Further, a 72-kDa band was detected when cell membrane fractions were analysed with these antisera (ASCP) by solid phase immunoisolation technique and immunoblot analysis. The ASCP is not an IgA-binding receptor, since anti-SC did not block either mIgA or pIgA binding. Further, the effects of proteolytic enzymes were different on these three IgA-binding molecule candidates. FcαR and ASCP were shown to be sensitive to pronase proteolytic degradation, but were resistant to trypsin and trypsin/EDTA treatments. In contrast, β-GT was sensitive to both pronase and trypsin treatments. We conclude that multiple IgA receptors are present on mouse T cells, and include those which bind to mIgA or dIgA (FcαR) as well as those which bind to pIgA (β-GT). Further, ASCP is also present on mouse T-cell lines, but its role in IgA binding to T cells remains to be further determined.