F1000 review:
Marcia Blackman
Trudeau Institute, United States of AmericaThis important study combines cellular immunology and intravital two-photon imaging to define the role of SAP (signalling lymphocyte activation molecule-associated protein) in interactions between B and T cells that are essential for germinal center reactions. These data enhance our understanding of mechanisms underlying impaired humoral immunity in SAP-deficient hosts and also elucidate the molecular basis of distinct interactions between CD4 T cells and B cells versus dendritic cells. The results have implications for human disease, in that deficient interactions between CD4 T cells and B cells are consistent with immunodeficiencies associated with the X-linked lymphoproliferative syndrome and enhanced interactions might contribute to abnormal antibody responses characteristic of autoimmune disease.
Pubmed Abstract:
Nature. 2008 Oct 9;455(7214):764-9.
SAP-controlled T-B cell interactions underlie germinal centre formation.Qi H, Cannons JL, Klauschen F, Schwartzberg PL, Germain RN.
Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Generation of long-term antibody-mediated immunity depends on the germinal centre reaction, which requires cooperation between antigen-specific T and B lymphocytes. In human X-linked lymphoproliferative disease and its gene-targeted mouse model, loss-of-function mutations in signalling lymphocyte activation molecule-associated protein (SAP, encoded by SH2D1a) cause a profound defect in germinal centre formation by an as yet unknown mechanism. Here, using two-photon intravital imaging, we show that SAP deficiency selectively impairs the ability of CD4(+) T cells to stably interact with cognate B cells but not antigen-presenting dendritic cells. This selective defect results in a failure of antigen-specific B cells to receive adequate levels of contact-dependent T-cell help to expand normally, despite Sap(-/-) T cells exhibiting the known characteristics of otherwise competent helper T cells. Furthermore, the lack of stable interactions with B cells renders Sap(-/-) T cells unable to be efficiently recruited to and retained in a nascent germinal centre to sustain the germinal centre reaction. These results offer an explanation for the germinal centre defect due to SAP deficiency and provide new insights into the bi-directional communication between cognate T and B cells in vivo.
The reason why I am interested in this paper:
To visualize T–DC interactions (Fig. 1), 2 times 106 OVA323-pulsed DCs per mouse were injected subcutaneously 24 h before intravenous transfer of Sap+/+ and Sap-/- T cells (3 times 106 each). Imaging was conducted 12 to 24 h later. To examine activation phenotypes, 106 DCs and 2 times 105 GFP-expressing T cells per mouse were used. To visualize interactions between B cells and Sap+/+ and Sap-/- T cells in the same lymph node (Fig. 2), 3 times 106 OT-2 T cells of each genotype were co-transferred into mice together with 5 times 106 wild-type B cells. Immunization was performed 12 h before cell transfer, and imaging was conducted 24 to 36 h thereafter. To visualize T–B interactions under non-competitive conditions and to assay T and B cell expansion (Fig. 3), 6 times 104 GFP-expressing OT-2 T cells were co-transferred together with 3 times 105 CFP-expressing B cells 24 h before immunization. Imaging and cytometric analyses were conducted 60–72 and 96 h later, respectively. To visualize GC recruitment and retention of T cells (Fig. 4), 3 times 104 CFP-expressing Sap+/+ and 3 times 104 GFP-expressing Sap-/- OT-2 T cells were co-transferred with 3 times 105 non-fluorescent MD4 B cells. Imaging was conducted 6 to 8 days post immunization. Dye-labelled naive B cells (2–4 times 107) were given 1 day before imaging to provide follicle and GC landmarks. The imaging set-up was essentially as described previously44. For imaging sessions longer than 2 h, the animal's hydration was maintained by lactated Ringer's solution given by means of a catheter. The typical x times y times z dimension was 0.5–1.1 times 0.5–1.1 times 3 microm, and the time resolution was 30–45 s. For experiments involving co-transfer of two types of dye-labelled T cells, the cells were always reciprocally labelled to control for potential dye-induced behavioural differences.
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