Expression of error-prone polymerases in BL2 cells activated for Ig somatic hypermutation

High affinity antibodies are generated in mice and humans by means of somatic hypermutation (SHM) of variable (V) regions of Ig genes. Mutations with rates of 10^-5-10^-3 per base pair per generation, about 10⁶-fold above normal, are targeted primarily at V-region hot spots by unknown mechanisms. We have measured mRNA expression of DNA polymerases ι, η, and ζ by using cultured Burkitt's lymphoma (BL)2 cells. These cells exhibit 5-10-fold increases in heavy-chain V-region mutations targeted only predominantly to RGYW (R = A or G, Y = C or T, W = T or A) hot spots if costimulated with T cells and IgM crosslinking, the presumed in vivo requirements for SHM. An ∼4-fold increase pol ι mRNA occurs within 12 h when cocultured with T cells and surface IgM crosslinking. Induction of pols η and ζ occur with T cells, IgM crosslinking, or both stimuli. The fidelity of pol ι was measured at RGYW hot-and non-hot-spot sequences situated at nicks, gaps, and double-strand breaks. Pol ι formed T·G mispairs at a frequency of 10^-2, consistent with SHM-generated C to T transitions, with a 3-fold increased error rate in hot- vs. non-hot-spot sequences for the single-nucleotide overhang. The T cell and IgM crosslinking-dependent induction of pol ι at 12 h may indicate an SHM "triggering" event has occurred. However, pols ι, η, and ζ are present under all conditions, suggesting that their presence is not sufficient to generate mutations because both T cell and IgM stimuli are required for SHM induction.