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CD14: the silent reaper?
U P D AT E
I M M U N O L O G Y TO D AY
Current literature LT-␣-expressing B cells in development of lymphoid follicles
CD14: the silent reaper?
Gonzalez, M., Mackay, F., Browning, J.L., Kosco-Vilbois, M.H. and Noelle, R.J. (1998) The sequential role of lymphotoxin and B cells in the development of splenic follicles J. Exp. Med. 187, 997–1007 Fu, Y-X., Huang, G., Wang, Y. and Chaplin, D.D. (1998) B lymphocytes induce the formation of follicular dendritic cell clusters in a lymphotoxin a-dependent fashion J. Exp. Med. 187, 1009–1018
Devitt, A., Moffatt, O.D., Raykundalia, C. et al. (1998) Human CD14 mediates recognition and phagocytosis of apoptotic cells Nature 392, 505–509
The splenic lymphoid compartment is located in the white pulp, a cuff of lymphocytes lining the splenic arterioles, referred to as the periarteriolar lymphocyte sheath (PALS). The PALS comprises mainly T cells whereas the B cells form spheric primary follicles located in the periphery of the PALS. Following immunization, these primary follicles transform into secondary follicles, otherwise known as germinal centres (GCs), in which the process of immunoglobulin (Ig) affinity maturation and class switch occurs. The T- and B-cell areas in the white pulp are also differentiated from each other by accessorycell content: interdigitating dendritic cells (IDCs) are present in the PALS whereas follicular dendritic cells (FDCs) are associated with the B-cell follicles. The establishment and maintenance of the normal splenic architecture is a poorly understood process. It is known that cytokines of the tumour necrosis factor (TNF)/lymphotoxin (LT) family are important since knockout mice with defects in TNF/LT signalling have abnormal splenic architecture. These two reports examine the role of TNF/LT signals in development of splenic follicles and conclude that expression of LT-␣ on B cells drives the maturation and clustering of FDCs during development of primary follicles and germinal centres. Gonzales and colleagues used severe combined immunodeficiency (SCID) mice, which lack mature T cells, B cells and FDCs, to examine the role of LT in development and maintenance of normal splenic architecture. SCID mice have a small and partially differentiated white pulp containing marginal zone and IDCs, but lacking FDCs. Wild-type spleen cells can segregate into defined T- and B-cell areas shortly after their injection into SCID mice, and induce the appearance of host-derived FDCs. This process that can be blocked by injecting SCID recipients with a chimeric protein (LT-R–Ig) com-
posed of the LT- receptor (LT-R) fused to the Ig Fc region. Experiments using T or B cells from LT-␣ knockout (LT-␣⫺/⫺) mice during adoptive transfer suggest that it is LT-␣ expression on the B cells that is critical to induction of FDC development and splenic organization, although a contribution of LT-␣ expression on a minor non-T, non-B-cell component cannot be conclusively ruled out. The paper by Fu and colleagues takes this further by examining GC formation in LT-␣⫺/⫺ mice, which constitutively lack FDC clusters and GCs in peripheral lymphoid tissues, and are unable to undergo Ig class switch. As above, although both T and B cells are required to reconstitute development of splenic primary lymphoid architecture, it is the LT-␣-expressing B-cell component of spleen cell mixtures that is important for induction of FDC clusters. LT-␣ expression on T cells is irrelevant in this context, and LT-␣-expressing B cells can induce FDC cluster formation and GC development in the absence of T cells. GCs thus derived in the absence of T cells can subsequently function in a secondary response to sheep red blood cells (SRBCs), including Ig-class switching, once supplemental T cells are provided. The formation of FDC clusters and GCs takes at least 2–4 weeks, with mature IgG responses possible after that time. The function of normal primary and secondary lymphoid follicles is clearly dependent on a complex interaction between T cells, B cells and FDCs, with a key role for TNF/LT-mediated signals. The two studies described above show that a key role in development of primary and secondary follicles, especially the formation of FDC clusters, can be ascribed to the B-cell compartment. Moreover, it is likely that the dependence on LT-mediated signals is due, at least in part, to the essential role played by LT-␣-expressing B cells in this process.
CD14, a GPI-linked membrane glycoprotein, is known to bind bacterial LPS and thus trigger inflammatory responses. In fact, overstimulation via CD14 can cause lethal toxic shock syndrome. However, Devitt et al. show that CD14 can mediate a non-inflammatory response, acting as a receptor on phagocytic cells that facilitates clearance of apoptotic cells without triggering the release of proinflammatory cytokines. A monoclonal antibody (mAb), 61D3 had been shown to bind to the surface of monocyte-derived macrophages and inhibit their interaction with leukocytes undergoing apoptosis. 61D3 was used to screen a cDNA library derived from the HL60 promyelocytic leukaemia cell line, and identified a 55 kDa protein with characteristics of CD14. This was confirmed by showing that 61D3 reacted specifically with purified recombinant CD14. When transiently expressed in COS cells, CD14 facilitated binding to chromatin-dense, apoptotic lymphoma cells and subsequent apoptosis. This suggests that macrophage expression of CD14 is not the only context for promotion of phagocytosis. However, despite overexpression on COS cells, phagocytosis was not always observed, suggesting the involvement of additional, variably expressed surface molecules. 61D3 inhibited binding and phagocytosis, both by CD14transfected COS cells and macrophages. Similarly effective was a mAb, MEM-18, specific for the LPS-binding epitope on CD14. However, the pro-phagocytic function of CD14 can be separated from its LPS-binding activity in that the former does not result in release of proinflammatory cytokines such as TNF-␣. CD14 thus seems to be a multi-functional receptor with the capacity to interact with ‘infectious’ non-self components such as LPS as well as pro-apoptotic ‘self’ markers. It is not clear what the pro-phagocytic ligand on apoptotic cells is, nor whether it actively suppresses the elaboration of proinflammatory cytokines from macrophages, or merely avoids their release.
J U N E Vo l . 1 9
1 9 9 8 No.6
243
I M M U N O L O G Y TO D AY
Current literature LT-␣-expressing B cells in development of lymphoid follicles
CD14: the silent reaper?
Gonzalez, M., Mackay, F., Browning, J.L., Kosco-Vilbois, M.H. and Noelle, R.J. (1998) The sequential role of lymphotoxin and B cells in the development of splenic follicles J. Exp. Med. 187, 997–1007 Fu, Y-X., Huang, G., Wang, Y. and Chaplin, D.D. (1998) B lymphocytes induce the formation of follicular dendritic cell clusters in a lymphotoxin a-dependent fashion J. Exp. Med. 187, 1009–1018
Devitt, A., Moffatt, O.D., Raykundalia, C. et al. (1998) Human CD14 mediates recognition and phagocytosis of apoptotic cells Nature 392, 505–509
The splenic lymphoid compartment is located in the white pulp, a cuff of lymphocytes lining the splenic arterioles, referred to as the periarteriolar lymphocyte sheath (PALS). The PALS comprises mainly T cells whereas the B cells form spheric primary follicles located in the periphery of the PALS. Following immunization, these primary follicles transform into secondary follicles, otherwise known as germinal centres (GCs), in which the process of immunoglobulin (Ig) affinity maturation and class switch occurs. The T- and B-cell areas in the white pulp are also differentiated from each other by accessorycell content: interdigitating dendritic cells (IDCs) are present in the PALS whereas follicular dendritic cells (FDCs) are associated with the B-cell follicles. The establishment and maintenance of the normal splenic architecture is a poorly understood process. It is known that cytokines of the tumour necrosis factor (TNF)/lymphotoxin (LT) family are important since knockout mice with defects in TNF/LT signalling have abnormal splenic architecture. These two reports examine the role of TNF/LT signals in development of splenic follicles and conclude that expression of LT-␣ on B cells drives the maturation and clustering of FDCs during development of primary follicles and germinal centres. Gonzales and colleagues used severe combined immunodeficiency (SCID) mice, which lack mature T cells, B cells and FDCs, to examine the role of LT in development and maintenance of normal splenic architecture. SCID mice have a small and partially differentiated white pulp containing marginal zone and IDCs, but lacking FDCs. Wild-type spleen cells can segregate into defined T- and B-cell areas shortly after their injection into SCID mice, and induce the appearance of host-derived FDCs. This process that can be blocked by injecting SCID recipients with a chimeric protein (LT-R–Ig) com-
posed of the LT- receptor (LT-R) fused to the Ig Fc region. Experiments using T or B cells from LT-␣ knockout (LT-␣⫺/⫺) mice during adoptive transfer suggest that it is LT-␣ expression on the B cells that is critical to induction of FDC development and splenic organization, although a contribution of LT-␣ expression on a minor non-T, non-B-cell component cannot be conclusively ruled out. The paper by Fu and colleagues takes this further by examining GC formation in LT-␣⫺/⫺ mice, which constitutively lack FDC clusters and GCs in peripheral lymphoid tissues, and are unable to undergo Ig class switch. As above, although both T and B cells are required to reconstitute development of splenic primary lymphoid architecture, it is the LT-␣-expressing B-cell component of spleen cell mixtures that is important for induction of FDC clusters. LT-␣ expression on T cells is irrelevant in this context, and LT-␣-expressing B cells can induce FDC cluster formation and GC development in the absence of T cells. GCs thus derived in the absence of T cells can subsequently function in a secondary response to sheep red blood cells (SRBCs), including Ig-class switching, once supplemental T cells are provided. The formation of FDC clusters and GCs takes at least 2–4 weeks, with mature IgG responses possible after that time. The function of normal primary and secondary lymphoid follicles is clearly dependent on a complex interaction between T cells, B cells and FDCs, with a key role for TNF/LT-mediated signals. The two studies described above show that a key role in development of primary and secondary follicles, especially the formation of FDC clusters, can be ascribed to the B-cell compartment. Moreover, it is likely that the dependence on LT-mediated signals is due, at least in part, to the essential role played by LT-␣-expressing B cells in this process.
CD14, a GPI-linked membrane glycoprotein, is known to bind bacterial LPS and thus trigger inflammatory responses. In fact, overstimulation via CD14 can cause lethal toxic shock syndrome. However, Devitt et al. show that CD14 can mediate a non-inflammatory response, acting as a receptor on phagocytic cells that facilitates clearance of apoptotic cells without triggering the release of proinflammatory cytokines. A monoclonal antibody (mAb), 61D3 had been shown to bind to the surface of monocyte-derived macrophages and inhibit their interaction with leukocytes undergoing apoptosis. 61D3 was used to screen a cDNA library derived from the HL60 promyelocytic leukaemia cell line, and identified a 55 kDa protein with characteristics of CD14. This was confirmed by showing that 61D3 reacted specifically with purified recombinant CD14. When transiently expressed in COS cells, CD14 facilitated binding to chromatin-dense, apoptotic lymphoma cells and subsequent apoptosis. This suggests that macrophage expression of CD14 is not the only context for promotion of phagocytosis. However, despite overexpression on COS cells, phagocytosis was not always observed, suggesting the involvement of additional, variably expressed surface molecules. 61D3 inhibited binding and phagocytosis, both by CD14transfected COS cells and macrophages. Similarly effective was a mAb, MEM-18, specific for the LPS-binding epitope on CD14. However, the pro-phagocytic function of CD14 can be separated from its LPS-binding activity in that the former does not result in release of proinflammatory cytokines such as TNF-␣. CD14 thus seems to be a multi-functional receptor with the capacity to interact with ‘infectious’ non-self components such as LPS as well as pro-apoptotic ‘self’ markers. It is not clear what the pro-phagocytic ligand on apoptotic cells is, nor whether it actively suppresses the elaboration of proinflammatory cytokines from macrophages, or merely avoids their release.
J U N E Vo l . 1 9
1 9 9 8 No.6
243