The Role of ERK5 Signaling in Tolerance Induction by Veto CTLs.
Authors: Reich-Zeliger Shlomit , Hanoch Tamar , Seger Rony , Reisner Yair 
Published: April 2020

Abstract
Several bone marrow cells and lymphocyte subpopulations, known as ‘veto cells’, were shown to induce transplantation tolerance across major histocompatibility antigens. Recently, it has been suggested that anti-3rd party CTLs depleted of alloreactivity against the host are endowed with marked veto activity and can facilitate bone marrow allografting. The veto mechanism is still obscure. While early studies emphasized the role of CD8 mediated apoptosis, we showed that the veto activity of anti-3rd party CD8+ CTLs is dependent upon the simultaneous expression of both CD8 and FasL. Thus, it seems that although Fas is upregulated on the effector T cells upon engagement of their TCR by class I of the veto cells, the presence of FasL on the veto cells cannot result in apoptosis of the effector cells unless CD8 on the veto cells is available and can interact with class 1 on the effector cells. Thus, the interaction of CD8 on the veto cells with class I on the effector cells seems to be associated with an increased susceptability of the effector cells to FasL killing. To further delineate the mechanism of the veto effect we have now studied the role of different signaling pathways using specific inhibitors. Spleen CD8 T cells from 2C mice (H2b) bearing TCR transgene directed against (H2d) were used as effector cells and anti FVB (H2q, third party) CTLs generated from Balb/c spleen cells (H2d) were used as veto cells. The addition of the latter cells to MLR of 2C against Balb/c (H2d) simulators, leads to deletion of the 2C effector CD8 cells within 72 hrs. Deletion monitored by FACS analysis of cells stained with anti-TCR transgene antibody (1B2+) revealed reduction from 46%±11% 1B2+CD8+ cells to 6%±3% 1B2+CD8+ cells in 6 different experiments. In contrast, veto CTLs generated from SJL (H2s) spleen cells that do not display the H2 recognized by the 2C effector cells, did not result in a significant deletion of the effector cells (42%±12% 1B2+CD8+ cells in 6 different experiments). The specific deletion exhibited by veto CTLs of Balb/c origin, can be inhibited by MEK1/2/5 inhibitors such as U0126, reducing the veto activity from 85.5%±7% to 16%±14% in 6 different experiments. The effective concentration of U0126 was relatively high (10μM), and lower concentration of this drug (1μM) had no response, indicating a potential involvement of the MEK5/ERK5 cascade rather than the MEK1/2-ERK1/2 cascade, in the veto effect. In addition, no inhibition of veto activity could be found with specific inhibitors of other signaling molecules such as JNK, P38, PI3K or PKC. Considering that Fas expression on the effector cells is critical for the veto activity, it is interesting that the ERK inhibitor didn’t affect the level of Fas on the effectors (93%±3% of 1B2+CD8+ upregulate FAS in the presence of U0126 in 7 different experiments). Also, this inhibition is not likely mediated by affecting the veto CTLs, as pretreatment of the latter cells with ERK inhibitor didn’t diminish the veto effect. The pro-apoptotic effects of MEK5-ERK5 cascade in this system is intriguing because these kinases are usually thought to promote proliferation and survival in most cellular systems. Therefore, the veto cells exhibit a unique signaling system, which utilizes ERK5 cascade to induce apoptosis. Further studies are directed at the potential links between the ERK5 cascade, the Fas system and the rest of the apoptotic machinery.