Ig isotype switching in B lymphocytes: The effect of T cell-derived interleukins, cytokines, cholera toxin, and antigen on isotype switch frequency of a cloned B cell lymphoma

The murine B cell lymphoma CH12.LX, which bears cell surface IgM specific for the phosphatidyl choline epitope of sheep red blood cells, is capable of spontaneous isotype switching in vitro. Switching to IgG3, IgG1, IgG2b, and IgA has been observed and variants expressing those isotypes have been isolated and cloned. We have developed a procedure for precise numerical evaluation of the frequency of switching to the several isotypes to which CH12.LX can switch. We have used a modified Poisson method which can distinguish between treatments which change isotype switch frequency and those which affect, in an isotype-specific fashion, growth or secretion rates of cells which have already switched. In this report we examine the effect of several cytokines, cholera toxin, hydroxyurea, and antigen on the isotype switch frequency of CH12.LX. The strongest effect observed was that of transforming growth factor-β, which increases switch frequency 40-fold to an absolute switch frequency of 0.04 switch events (from IgM to IgA expression) per cell division. Interleukin-4 (IL-4) and cholera toxin also increase the switch frequency of CH12.LX while IL-5, IL-6 (with or without antigen), antigen (SRBC) alone, interferon-γ, or hydroxyurea have no effect. We have shown that none of the cytokines studied change the relative frequency of switching to the available isotypes, only the absolute frequency of switching. We infer from this that the factors tested do not 'instruct' CH12.LX to switch to a particular isotype, but rather they deliver a 'go' signal to cells committed to switching to IgA at high frequency, rarely to IgG3, IgG1, or IgG2b, and never to IgG2a or IgE.