Is Toso/Igm Fc Receptor (Fc Mu R) Expressed By Innate Immune Cells?

Lang et al. have recently published that innate immune cells, including bone-marrow granulocytes and splenic macrophages, express the plasma-membrane protein Toso as a unique regulator of innate immune responses during bacterial infection and septic shock (1). In contrast, we and others have previously reported that the IgM Fc receptor (FcμR), originally designated as Toso or Fas apoptosis inhibitory molecule 3, is expressed only by B-lineage cells in mice and is involved in IgM homeostasis, B-cell survival, and humoral immune responses (2, 3). Because of these conflicting results, we have extensively reexamined the cellular distribution of Toso/FcμR and wish to provide some new results and comments. First, we conducted flow cytometric analyses of cell-surface Toso/FcμR. Cells were first incubated with two FcγR-blocking monoclonal antibodies (mAbs), a critical step when staining myeloid cells, and then with a panel of five different murine mAbs with proven specificity for mouse Toso/FcμR (2), followed by fluorochrome-labeled rat anti-mouse κ mAb as the detection reagent. None of the anti-FcμR mAbs demonstrated specific cell-surface staining, compared with their isotype-matched control mAbs, of immature, mature granulocytes or myelomonocytoid cells from wild-type bone-marrow (Fig. 1A). In addition, Toso/FcμR was not expressed on granulocytes or macrophages from wild-type spleen (Fig. 1B). When the cell-surface staining of Toso/FcμR was analyzed using Fcmr-deficient cells as a negative control, as was done by Lang …