To determine the immunologic effects of anti–CD40 ligand (anti‐CD40L) therapy in 5 patients with systemic lupus erythematosus nephritis who participated in an open‐label study of a humanized anti‐CD40L monoclonal antibody.

Serum and peripheral blood mononuclear cells were obtained before, during, and after treatment, and the frequency of Ig and anti‐DNA antibody–secreting B cells was analyzed by enzyme‐linked immunospot assay and by analysis of Epstein‐Barr virus (EBV)–transformed B cell lines. To determine the effect of treatment on somatic mutation of Ig genes, reverse transcriptase–polymerase chain reaction was performed on messenger RNA from 4 patients, using primers specific for the DP‐47 heavy chain gene and for IgG. Finally, B cell phenotype was investigated using flow cytometry.

Even a brief period of treatment with anti‐CD40L markedly reduced the frequency of IgG and IgG anti‐DNA antibody–producing B cells, and these changes persisted for several months after cessation of treatment. To confirm these findings, EBV‐transformed B cell lines were screened from each of 3 patients, and a 10‐fold decrease in anti‐DNA antibody–secreting cell lines was found after treatment in all 3 patients. Few differences in mutation patterns were observed before and after treatment; however, the frequency of germline‐encoded DP‐47 sequences was significantly increased before treatment and normalized following treatment. Flow cytometric analysis of B cells revealed expansion of a CD27−/IgD− B cell subset in some of the patients, which did not change with treatment.

These are the first mechanistic studies of the effect of anti‐CD40L therapy in human autoimmune disease. The results suggest that further studies of CD40L blockade are warranted.