B-Cell Responses

BCL6 controls the expression of the B7-1/CD80 costimulatory receptor in germinal center B cells. Niu, H. et al. J. Exp. Med. 198, 211–221 (2003)

In this study, Dalla-Favera and colleagues looked at the targets of transcriptional repression by BCL6 to gain a better understanding of its involvement in germinal-centre (GC) formation. Previous studies have not been able to distinguish direct targets of BCL6 from secondary effects. This study shows that BCL6 directly inhibits transcription of the CD80 gene by binding to its promoter enhancer and prevents the upregulation of expression of CD80 in response to CD40–CD40L interactions mediated by nuclear factor-κB (NF-κB). CD80 has a crucial role in T-cell–B-cell interactions required for GC development and T-cell-dependent antibody responses. Therefore, constitutive expression of BCL6 owing to a translocation might lead to lymphomagenesis by disrupting the normal GC differentiation pathway of B cells.

Monocytes

Blood monocytes consist of two principal subsets with distinct migratory properties. Geissmann, F. et al. Immunity 19, 71–82 (2003)

Two subsets of human blood monocytes that differ in their phenotypic and functional characteristics were identified several years ago, but it has proven difficult to characterize any such subsets in mice. Now, Geissmann et al. describe two mouse monocyte subsets with distinct chemokine-receptor and adhesion-molecule expression profiles, which correspond to the human subsets. CC-chemokine receptor 2 (CCR2)-positive monocytes are short-lived cells that are recruited to inflamed tissues, whereas the CCR2-negative subset is recruited to non-inflamed tissues and these cells might be the precursors of tissue-resident macrophages and dendritic cells. Both subsets have the capacity to develop into dendritic cells. Further characterization of monocyte subsets in mice should contribute to an increased understanding of the role of monocytes in human diseases.

Splenic Architecture

A conduit system distributes chemokines and small blood-borne molecules through the splenic white pulp. Nolte, M. A. et al. J. Exp. Med. 198, 505–512 (2003)

The splenic white pulp is structurally similar to a lymph node, although the spleen receives antigen directly from the blood and not through afferent lymphatics. This paper shows that, similar to the lymph node, the spleen has a conduit system — a kind of structured transport system. The splenic conduit is a tubular network containing collagen fibres that are surrounded by reticular fibroblasts. The white pulp is known to restrict cellular movement to lymphocytes and dendritic cells, but this study shows that the conduit also limits the entry of large molecules. Locally produced chemokines were also found in the conduit system — CC-chemokine ligand 21 (CCL21) in the T-cell area and CXCL13 in the B-cell area.