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Original Article |
Correspondence to: Frederick W. Alt, The Howard Hughes Medical Institute, the Children's Hospital, Enders 861, 300 Longwood Ave., Boston, MA 02115. Tel:617-355-7290 Fax:617-738-0163 E-mail:alt{at}rascal.med.harvard.edu.
Splenic B lineage cells expressing recombination activation genes (RAG+) in mice immunized with 4-hydroxy-3-nitrophenyl-acetyl coupled to chicken 
-globulin (NP-CGG) and the adjuvant aluminum-hydroxide (alum) have been proposed to be mature B cells that reexpress RAG after an antigen encounter in the germinal center (GC), a notion supported by findings of RAG expression in peripheral B lymphocyte populations activated in vitro. However, recent studies indicate that these cells might be immature B cells that have not yet extinguished RAG expression. Here, we employ RAG2–green fluorescent protein (GFP) fusion gene knock-in mice to show that RAG+ B lineage cells do appear in the spleen after the administration of alum alone, and that their appearance is independent of T cell interactions via the CD40 pathway. Moreover, splenic RAG+ B lineage cells were detectable in immunized RAG2-deficient mice adoptively transferred with bone marrow (BM) cells, but not with spleen cells from RAG+ mice. Although splenic RAG+ B cells express surface markers associated with GC B cells, we also find the same basic markers on progenitor/precursor BM B cells. Finally, we did not detect RAG gene expression after the in vitro stimulation of splenic RAG- mature B cells with mitogens (lipopolysaccharide and anti-CD40) and cytokines (interleukin [IL]-4 and IL-7). Together, our studies indicate that RAG+ B lineage cells from BM accumulate in the spleen after immunization, and that this accumulation is not the result of an antigen-specific response.
Key Words: alum, germinal center, receptor editing, immunoglobulin re-rearrangement
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