EFFICIENT RECOMBINATION OF A SWITCH SUBSTRATE RETROVECTOR IN CD40-ACTIVATED B-LYMPHOCYTES - IMPLICATIONS FOR THE CONTROL OF C-H GENE SWITCHRECOMBINATION

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 ab out B lymphocyte biology and development, Class switch recombination i s controlled by cytokine response elements that are required to differ entially activate C-H gene transcription before their subsequent recom bination. Here, we show that cultures of purified murine and human B c ells, stimulated only by CD40 receptor engagement, possess a potent sn itch recombination activity, CD40 ligand-stimulated murine and human B lymphocytes were infected with recombinant retroviruses containing S mu and S gamma 2b sequences. Chromosomally integrated switch substrate retrovectors (SSRs), harboring constitutively transcribed S sequences , underwent extensive recombinations restricted to their S sequences w ith structural features akin to endogenous switching. SSR recombinatio n commenced 4 days postinfection (5 days poststimulation) with extensi ve switch sequence recombination over the nest 2 to 3 days. In contras t, endogenous S gamma 2b and S gamma 1 sequences did not undergo appre ciable switch recombination upon CD40 signaling alone. As expected, IL -4 induced endogenous S mu. to S gamma 1 switching, while endogenous S mu to S gamma 2b fusions remained undetectable, Surprisingly, IL-4 en hanced the onset of SSR recombination in CD40-stimulated murine B cell s, with S-S products appearing only 2 days postinfection and reaching a maximum within 2 to 3 days, The efficiency of switch recombination w ith SSRs ressembles that seen for endogenous C-H class switching.