Efficient Recombination of a Switch Substrate Retrovector in CD40-Activated B Lymphocytes: Implications for the Control of CH Gene Switch Recombination

Maturing B lymphocytes possess a recombination activity that switches the class of heavy chain Ig. The nature of the recombination activity, its molecular requirements and regulation remain elusive questions about B lymphocyte biology and development. Class switch recombination is controlled by cytokine response elements that are required to differentially activate CH gene transcription before their subsequent recombination. Here, we show that cultures of purified murine and human B cells, stimulated only by CD40 receptor engagement, possess a potent switch recombination activity. CD40 ligand-stimulated murine and human B lymphocytes were infected with recombinant retroviruses containing Sμ and Sγ2b sequences. Chromosomally integrated switch substrate retrovectors (SSRs), harboring constitutively transcribed S sequences, underwent extensive recombinations restricted to their S sequences with structural features akin to endogenous switching. SSR recombination commenced 4 days postinfection (5 days poststimulation) with extensive switch sequence recombination over the next 2 to 3 days. In contrast, endogenous Sγ2b and Sγ1 sequences did not undergo appreciable switch recombination upon CD40 signaling alone. As expected, IL-4 induced endogenous Sμ to Sγ1 switching, while endogenous Sμ to Sγ2b fusions remained undetectable. Surprisingly, IL-4 enhanced the onset of SSR recombination in CD40-stimulated murine B cells, with S-S products appearing only 2 days postinfection and reaching a maximum within 2 to 3 days. The efficiency of switch recombination with SSRs ressembles that seen for endogenous CH class switching.