T-CELL CLONES FROM AN X-LINKED HYPER-IMMUNOGLOBULIN (IGM) PATIENT INDUCE IGE SYNTHESIS IN-VITRO DESPITE EXPRESSION OF NONFUNCTIONAL CD40 LIGAND

The induction of immunoglobulin E (IgE) switching in B cells requires at least two signals. The first is given by either of the soluble lymp hokines interleukin 4 (IL-4) or IL-13, whereas the second is contact d ependent. It has been widely reported that a second signal can be prov ided by the CD40 ligand (CD40L) expressed on the surface of T cells, m ast cells, and basophils. A defect in the CD40L has been shown recentl y to be responsible for the lack of IgE, IgA, and IgG, characteristic of the childhood X-linked immunodeficiency, hyper IgM syndrome (HIGM1) . IgE can however be detected in the serum of some HIGM1 patients. In this study, we isolated T cell clones and lines using phytohemagglutin in (PHA) and allergen, respectively, from the peripheral blood of one such patient who expressed a truncated form of CD40L, and investigated their ability to induce IgE switching in highly purified, normal tons illar B cells in vitro. Unexpectedly, 4 of 12 PHA clones tested induce d contact-dependent IgE synthesis in the presence of exogenous IL-4. T hese clones were also shown to strongly upregulate IL-4-induced germli ne E RNA and formed dense aggregates with B cells. Of the four helper clones, three were CD8(+), of which two were characteristic of the T h elper cell 2 (Th2) subtype. Two allergen-specific HIGM1 T cell lines, both of the Th0 subtype, could also drive IgE synthesis when prestimul ated using specific allergen, All clones and lines were negative for s urface expression of CD40L, and the mutated form of CD40L was confirme d for a representative clone by RNase protection assay and sequencing. The IgE helper activity could not be attributed to membrane tumor nec rosis factor alpha (TNF-alpha) although it was strongly expressed on a ctivated clones, and the addition of neutralizing anti-TNF-alpha antib ody did not abrogate IgE synthesis. These results therefore suggest th e involvement of T cell surface molecules other than CD40L in the indu ction of IgE synthesis, and that these molecules may also be implicate d in other aspects of T-B cell interactions.