To the editor:
Since the publication by Nguyen et al1 showing that Toso/FcμR is an inhibitor of Fas-mediated apoptosis, there has been debate regarding whether FcμR regulates apoptosis.2-4 An obvious problem is that these original studies used cell lines instead of normal primary T and B cells. To clarify the physiological role of FcμR in Fas-mediated apoptosis, we have now used FcμR-deficient mouse T and B cells. Our results clearly show that FcμR is not involved in Fas-mediated cell death in thymocytes or in spleen T and B cells before and after activation.
We first analyzed cell-surface expression of Fas, Fas ligand (FasL), and FcμR by wild-type (WT) and FcμR-deficient cells (Figure 1A). Thymocytes and spleen T and B cells before and after activation all expressed Fas, and WT and FcμR−/− cells had similar levels of expression (Figure 1A top panels). FasL was only expressed by activated T cells and again there was no difference in the expression levels between WT and FcμR−/− cells (Figure 1A middle panels). Consistent with our recent studies,5,6 FcμR was only expressed by B cells but not by thymocytes or T cells before and after activation (Figure 1A bottom panels).
We next examined Fas-mediated cell death in thymocytes and T cells. As shown previously,7 thymocytes were readily sensitive to cell death induced by either anti-Fas (Jo-2) or FasL, and WT and FcμR−/− thymocytes showed similar Fas susceptibility (Figure 1B). In addition, WT and FcμR−/− spleen T cells showed the same sensitivity to FasL-induced cell death (Figure 1C left panel). Activated T cells are known to undergo an activation-induced cell death (AICD) mediated by Fas/FasL interaction,8 and WT and FcμR−/− T cells showed similar AICD after CD3 stimulation (Figure 1C middle panel). Treatment of anti-CD3–activated T cells with FasL further increased cell death and, again, WT and FcμR−/− T cells were equally sensitive (Figure 1C right panel). These results demonstrate that WT and FcμR−/− thymocytes and spleen T cells before and after activation have similar sensitivity to Fas-mediated cell death, which is also consistent with the lack of FcμR expression by these cells (Figure 1A).
We then analyzed Fas-mediated cell death in B cells (Figure 1D). Consistent with earlier studies,9,10 spleen B cells were resistant to anti-Fas– or FasL-induced cell death (Figure 1D left 2 panels) and became susceptible after CD40 ligation (Figure 1D right 2 panels). Despite the fact that FcμR was only expressed by WT but not FcμR−/− B cells (Figure 1A), these cells exhibited similar susceptibility to cell death induced by either the anti-Fas mAb or FasL (Figure 1D right two panels). These results demonstrate that FcμR−/− B cells do not show elevated sensitivity to Fas-mediated cell death.
Based on the above results, we conclude that FcμR is NOT an inhibitor of Fas-mediated cell death in normal mouse T and B cells. Our results cannot exclude the possibility that FcμR may have a role in Fas-mediated apoptosis in human B and T cells. However, given the controversial results from different groups,1-4 further studies are required to clarify the role of FcμR in human B and T cells.
Acknowledgment: The authors thank Prof Peter Burrows for helpful comments.
Contribution: R.O. and H.M. performed the experiments; H.O. provided FcμR-deficient mice; and J.-Y.W. designed the experiments and wrote the paper.
Conflict-of-interest disclosure: The authors declare no competing financial interests.
Correspondence: Ji-Yang Wang, Laboratory for Immune Diversity, RIKEN Research Center for Allergy and Immunology, 1-7-22 Suhehiro-cho, Tsurumi, Yokohama 230-0045, Japan; e-mail: firstname.lastname@example.org.