Effect of transforming growth factor (TGF)-beta 1 on IgA isotype expression. TGF-beta 1 induces a small increase in sIgA+ B cells regardless of the method of B cell activation.
Transforming growth factor-beta (TGF-beta) has been reported to play an important role in IgA isotype expression when B cells are stimulated with LPS. The goal of the present study was to determine whether TGF-beta has similar effects on IgA isotype expression under more physiologic conditions utilizing a variety of B cell activation systems. As previously reported, in the LPS system TGF-beta caused a small, but significant, absolute increase in surface IgA (sIgA) expression and a very definite increase in IgA secretion; these effects were enhanced by IL-2 and IL-5. In the case of B cell stimulation with another B cell stimulant, the thymus-independent type II mitogen, anti-IgD-dextran, TGF-beta led to a similar small increase in sIgA expression, but caused suppression of IgA secretion. Using the Th2 cell clones CDC35 and D10 to stimulate resting B cells in a cognate and non-cognate T cell-dependent fashion, respectively, TGF-beta again increased sIgA expression to a similar small extent. TGF-beta at low doses (0.1 ng/ml) did not increase IgA secretion significantly and, at higher doses (1.0 ng/ml) caused significant suppression of IgA secretion. Addition of various cytokines (IL-2, -4, -5, D10sup) other than TGF-beta to stimulated B cells did not increase sIgA expression, but did give rise to increased amounts of IgA secretion, especially when activated D10 T cells were used as the B cell stimulant. Finally, the addition of an antibody against TGF-beta to cultures containing TGF-beta on day 2 led to partial or complete reversal of the inhibitory effects of TGF-beta on IgA secretion. In conclusion, TGF-beta causes a consistent, but small increase in sIgA+ B cells, in cultures of B cells stimulated by a variety of T cell-dependent or independent stimuli. In contrast, TGF-beta either promotes or inhibits B cell survival and IgA secretion, depending on the method of B cell activation. These results are most consistent with the view that TGF-beta provides only a partial or incomplete IgA switch signal but that additional factors are involved in IgA isotype switching and differentiation.