The purpose of this proposal is to determine how the different antibody classes or isotypes are regulated with emphasis on the role of the helper T cell. The majority of foreign antigens give rise to an antibody response diverse in isotypes. Therefore, it has been difficult to determine how isotypes can be individually regulated. However, some infectious organisms result in the production of one or two isotypes such as the parasite Mesocestoides corti which results in a dramatic increase in IgG1 with little or no effect on the other non-IgM isotypes. Using this physiological model systems, the control of IgG1 production will be studied at both cellular and molecular levels. Helper T cells clones derived from parasite infected animals appear to both enhance IgG1 and downregulate the remaining isotypes in vitro. Using cloned populations of cells and B cell limit dilution assays the mechanism of restriction to IgM and IgG1 will be delineated by determining the B cell target, the role of T cell derived soluble mediators, and the way in which the live parasite stimulates regulatory T cells. In this way, the mode of action of isotype restricting T cell clones will be determining which may involve mechanisms of induction, selection, and/or some type of suppression. In addition, the molecular mechanism of isotype restriction will be analyzed using a combination of in situ hybridization of single cells, Northern blot hybridizations, and Southern blot hybridization with several heavy chain specific probes. Moreover, the necessity for isotype switch recombination for IgG1 expression will be tested. These studies should contribute substantially to our overall understanding of antibody class expression as well as elucidate mechanism by which isotypes can be individually regulated. This is essential so that certain isotypes can be manipulated in disease states such as the IgE isotype which cause allergy.