2 LEVELS OF HELP FOR B-CELL ALLOANTIBODY PRODUCTION

We have examined whether T cell, stimulation by direct or indirect pat hways contributes to alloantibody production by B cells after major hi stocompatibility complex (MHC)-disparate skin graft rejection in mice. Experiments were performed using normal mice, MHC class II-deficient mice, MHC class II-deficient mice with an intact peripheral CD4(+) cel l population (due to expression of class II antigens only on thymic ep ithelium), mice lacking the cytoplasmic tail of their MHC class II ant igens, and mice depleted of CD4(+) cells by anti-CD4 monoclonal antibo dy treatment. Depletion of recipient CD4(+) cells reduced alloantibody production to barely detectable levels. Absence of donor MHC class II antigens did not affect the production of either immunoglobulin (Ig)M or IgG antibodies directed at class I alloantigens. Absence of recipi ent MHC class II antigens, however, led to production of only IgM but not IgG antibodies, even if the recipients had an intact CD4(+) cell p opulation. Absence of the cytoplasmic tail of the recipient's MHC clas s II antigens led to the production of slightly reduced amounts of IgG antibody. These findings indicate that (a) CD4(+) cells are essential helper cells for B cell alloantibody production; (b) production of Ig M alloantibody can occur with help from CD4(+) cells, which recognize either donor class II antigens or modified recipient class II antigens ; (c) isotype switching from IgM. to IgG alloantibody requires help fr om CD4(+) cells activated by antigens presented by recipient MHC class II molecules; and (d) the cytoplasmic domain of the recipient MHC cla ss II molecules may be involved in the mechanism that leads to isotype switching by B cells. Thus, there are two levels of CD4-mediated help available for B cells responding to alloantigens: one (involving a no ncognate interaction) can produce B cell activation, and a second (inv olving a cognate interaction) is required for differentiation and IgG alloantibody production.