In vivo allogeneic effects: Shift in the isotype profile of primary TI-2 responses in mice undergoing graft-vs-host reaction

We showed previously that primary responses to T-dependent (TD) and T-independent type 2 (TI-2) antigens were differentially affected by allogeneic effects induced in vivo during a graft-vs-host reaction (GVH). TD responses were ≥80% suppressed, whereas the TI-2 responses were greatly enhanced, particularly the IgG component, which normally is very low. We have analyzed the IgG subclass distribution in primary responses of normal and GVH F₁ mice in order to determine whether the strong T cell signals that occur during GVH reactions also induce shifts in the isotype profile. The effect of GVH on responses to TI-2 antigens was of particular interest because they are usually dominated by IgM and IgG3 classes in normal mice. We found a threefold to 10-fold increase in the PFC numbers of all four IgG subclasses in the response to TI-2 antigens, with an apparent shift from the usual IgG3 dominance to IgG1 in GVH mice. This IgG1 dominance was not found in serum antibodies where IgG3, IgG1, and IgG2b were equally expressed, although total IgG was increased > 20-fold. No isotype shift was found in either the TNP-KLH response, which was ≥75% suppressed (IgG1 dominance was retained) or in the TI-1 response to TNP-Ba. The latter response was reduced (25 to 50%) in GVH mice and continued to be dominated by IgG2b/2a and IgG3. Unlike the unique isotype patterns found in primary responses, TNP-KLH primed mice challenged with TD, TI-1, or TI-2 antigens gave memory responses with identical isotype profiles that were dominated by IgG1 PFC. The role of T cells in B cell differentiation and isotype expression is discussed.