Switching to IgG3, IgG2b, and IgA is division linked and independent, revealing a stochastic framework for describing differentiation

LPS was used to induce switching of B cells to IgG3 and, in the presence of TGF-beta, to IgG2b and IgA, Switching to all three isotypes increased with division number according to a consistent relationship that was independen t of time in culture. The mode of activation altered the relationship with division, as CD40 ligand increased switching to IgA and decreased switching to IgG2b and IgG3 when measured per division. This division-linked switchi ng behavior could be described by Gaussian probability distributions center ed around a mean division number. The divisions at which switching to IgG3 and IgG2b occurred overlapped, raising the possibility that the two switchi ng mechanisms were linked, However, when IgG3(+) and IgG3(-) B cells were s orted and placed back in-culture, they switched to IgG2b at an equivalent r ate; indicating that alternative switching decisions were made independentl y within a single cell. As a consequence, isotype switching could be predic ted at the population level by standard probability laws. Therefore, divisi on number provides a framework for a stochastic description of differentiat ion that may be widely applicable.