Systemic lupus erythematosus (SLE) is a generalized autoimmune disease 
characterized by abnormal B cell activation and the occurrence of increased 
frequencies of circulating plasma cells (PC). The molecular characteristics and 
nature of circulating PC and B cells in SLE have not been completely 
characterized. Microarray analysis of gene expression was used to characterize 
circulating PC in subjects with active SLE. Flow cytometry was used to sort PC 
and comparator B cell populations from active SLE blood, normal blood and normal 
tonsil. The gene expression profiles of the sorted B cell populations were then 
compared. SLE PC exhibited a similar gene expression signature as tonsil PC. The 
differences in gene expression between SLE PC and normal tonsil PC and tonsil 
plasmablasts (PB) suggest a mature Ig secreting cell phenotype in the former 
population. Despite this, SLE PC differed in expression of about half the genes 
from previously published gene expression profiles of normal bone marrow PC, 
indicating that these cells had not achieved a fully mature status. Abnormal 
expression of several genes, including CXCR4 and S1P(1), suggests a mechanism 
for the persistence of SLE PC in the circulation. All SLE B cell populations 
revealed an interferon (IFN) gene signature previously only reported in 
unseparated SLE peripheral blood mononuclear cells. These data indicate that SLE 
PC are a unique population of Ig secreting cells with a gene expression profile 
indicative of a mature, but not fully differentiated phenotype.