Identifying the expression of Mer among macrophage subpopulations in lymphatic organs will aid in understanding the role of Mer in the clearance of apoptotic cells, including lymphocytes. We have shown that Mer-KO mice develop a lupus-like autoimmune syndrome and splenomegaly. In addition, excessive apoptotic cells and cell debris were observed in Mer-KO mice spleens after exogenous infusion of apoptotic cells [8
]. Therefore, the anatomical localization and expression pattern of Mer in spleen are of particular interest. Mer seems to be universally expressed among CD68-bearing macrophages — so-called tingible body macrophages. These cells have been reported to be of critical importance in engulfing apoptotic debris, and, in lymph nodes from SLE patients, these cells have a marked deficiency in uptake of apoptotic cells [21
tingible body macrophages are found in splenic germinal centers, where they engulf newly generated apoptotic B cells. Our data suggest that Mer may be of special importance in the elimination of apoptotic B cells generated in the processes of negative selection and affinity maturation. In the absence of Mer, the build-up of these apoptotic B cells, which express nuclear antigens and other self proteins, may spur the development of autoimmunity by providing an antigenic stimulus for nearby autoreactive B cells. Together with the overactivated macrophages found in Mer-deficient mice, the potentially immunogenic apoptotic B cells may stimulate the spontaneous development of autoimmunity. Another important receptor/ligand system that facilitates uptake of apoptotic debris, milk fat globule–EGF factor 8 (MFG-E8)/lactadherin, is also centered on these tingible macrophages [22
], reinforcing the notion that they are central in apoptotic debris clearance.
The splenic marginal zone is the first area of the spleen encountered by circulating antigens and is believed to be of central importance in capturing antigens. Further, MZ B cells have been reported to contribute disproportionately to the production of autoantibodies, supporting a crucial role for the MZ in the genesis of autoimmunity. Given the importance of Mer in phagocytosis of apoptotic debris and in regulating macrophages activation, we were surprised to observe that Mer was not present on the metallophillic macrophages that are so important in the MZ. Instead. Mer was expressed on a small portion of MZ macrophages, which may represent a previously unrecognized MZ macrophage population. A proportion of these Mer+
MZ macrophages expresses MARCO, and may be functionally distinguishable from other MZ macrophages. The exact function of MARCO is unknown, but it has been suggested that it facilitates cell adhesion and shape change [23
]. Recently, a role for MARCO in the clearance of apoptotic cell was reported [24
]. Co-expression of Mer with MARCO might be necessary for the engulfment and antigen presentation of specific pathogens.
We addressed another important issue concerning how Mer functions in macrophages in vivo. This concerned the physiological ligand for Mer. While Gas6 was first identified as a ligand for Axl and subsequently a common ligand for all three members (Tyro-3, Axl, and Mer) within the same subfamily [5
], it is unclear whether this molecule is the only ligand and a strong case has been made for human protein S as an alternative ligand [17
]. A difficulty has been that all studies were done in a culture system involving bovine serum. Considering the cross-species activation of receptor by protein S [9
], care must be taken when interpreting data. Our data, using murine serum from wild type and from Gas6-KO mice, indicate that Gas6 is indeed the major ligand for Mer-mediated phagocytosis. Gas6-KO mice do not develop autoimmune disease (our unpublished data), despite their apparent inability to phagocytose apoptotic cells through Mer. This may indicate that Mer signaling for regulation of macrophage cytokine production may proceed through alternative pathways, viz., protein S, while phagocytosis of apoptotic cells through Mer largely depends on Gas6. These studies will require confirmation in other systems, but underline the complexity of the signaling through Mer and other related molecules in regulation of innate and adaptive immunity.