Depletion of CD4-effector memory T cells and clonally expanded IgG4 memory B cells may explain the therapeutic efficacy of rituximab in IgG4-related disease: Studies using flow cytometry and single-cell sequencing

Background/Purpose:

The clinical effectiveness of rituximab-mediated B cell depletion in chronic inflammatory disorders such as multiple sclerosis and Type I diabetes, which are not believed to be autoantibody-driven conditions, has not been explained satisfactorily. A mechanistic explanation is also lacking for the efficacy of rituximab in IgG4-related disease (IgG4-RD), a chronic inflammatory disorder characterized by fibrosis that might be T cell mediated.  Although CD4+ T cells contribute to the activation of B cells during T-B collaboration, activated B cells are required in turn both for the generation of T follicular helper cells and for the induction and maintenance of CD4+ memory T cells. We hypothesize that in IgG4-RD, some as yet unidentified antigens induce T follicular helper cell-mediated B cell activation and the subsequent generation of a Th2 type of CD4+ effector and memory  T cell response. The fibrosis that characterizes this disorder may reflect the generation of a B cell-dependent pathogenic Th2 cell response to these uncharacterized antigens.

Methods:

We performed detailed flow cytometry studies on B and T cell populations before and after treatment with rituximab in four patients, accompanied by single cell cloning studies of IgG4 memory B cells.

Results:

We have detected an expansion of circulating IgG4 memory B cells and of CD4+ effector memory T cells in subjects with active IgG4-RD (a, b). Rituximab therapy not only mediates the depletion of all circulating B cells in IgG4-RD, including the expanded IgG4 memory B cells, but also of the CD4+ effector memory T cell population.  Single cell cloning of IgG4 memory B cells from subjects with active disease followed by sequencing of matched Ig heavy and light chains reveals the possible expansion of a few IgG4 B cells with common V_H and V_L usage.

Conclusion:

These results support the notion that B cell depletion with rituximab mediates loss of the disease-promoting effector memory CD4+ T cells in IgG4-RD. These studies offer the possibility that the inciting antigen(s) in IgG4-RD may be identified in due course as a result of the identification of clonally-expanded B cells.