The Nitroalkene SANA Dampens Alloreactivity Through Heme Oxygenase-1 and Regulatory T Cells

Today, hardly anyone would dare claim that the immune system is simple to understand and that we, as biologists or clinicians, can easily manipulate it to achieve our ultimate aims. Autoimmune diseases represent so many unsolved questions, as do dysimmunity, allergies, and so on. However, access to increasingly advanced technologies is enabling us to discover totally unsuspected signaling pathways that impact the behavior of small groups of cells or even entire individuals. The puzzle is gigantic, and many pieces are missing. The immune system of living beings is modified by their environment and behavior. Imagine the encounter of an organ from a living or deceased donor transplanted into a genetically different recipient. Let us not forget to add the proinflammatory “adjuvant” effect of ischemia/reperfusion injury inherent in all organ transplants. All in all, it is a gigantic equation with multiple variables, and it is only beginning to take shape. Anyone wishing to find a linear and serene path must appeal either to the miraculous or to science. The latter inevitably requires exhaustive knowledge of the dominant pathways upstream and downstream of what has been defined over time as the “alloimmune response.” This is why the work of Galliussi et al1 makes such sense. By showing that the nitroalkene derivative of salicylate salicylic acid nitroalkene (SANA) can induce heme oxygenase-1 (HO-1) expression in conventional MHCII-CD11ChighCD11b+ dendritic cells, they brought a small but important stone in the building. Indeed, dendritic cells play a critical role in priming the alloimmune T-cell repertoire. In contrast, HO-1 expression by myeloid cells has been shown to dampen T-cell responses in several experimental models of alloreactivity or antitumoral responses, including previous works performed by the authors.2 HO-1 is a ubiquitous stress-induced cytoprotective enzyme with antioxidant and anti-inflammatory properties derived from breaking down heme compounds and generating biliverdin and bilirubin (antioxidants), carbon monoxide (a gas neurotransmitter with anti-inflammatory capacities), and free iron that will bind ferritin. Importantly, HO-1 transcription is regulated by the transcription factor Nrf2, which can be activated by the nitroalkene SANA in a TMEM176B-dependent manner and in an independent manner through the formation of reversible adducts. In addition to activating the hmox1 gene transcription, SANA also inhibits proinflammatory pathways such as the nuclear factor kappa B and the nucleotide-binding domain, leucine-rich–containing family, pyrin domain–containing-3 inflammasome.3 In short, the SANA compound gives MHCII-CD11ChighCD11b+ dendritic cells a more tolerogenic profile, which here is attested by a prolonged survival of fully allogeneic heart allografts in rats and prolonged survival of mouse minor antigen-mismatched skin grafts. In the latter, an increased infiltration of CD4+FoxP3+ regulatory T cells (Treg) was observed within the grafts. The administration of tin protoporphyrin IX (an inhibitor of HO-1 activity) abrogated the effect of SANA on Treg recruitment and skin graft survival. This suggests that HO-1 induction by SANA mobilizes Treg into the graft, which in turn controls rejection by CD8+ effector T cells, even if no formal experimental evidence of Treg involvement is provided (ie, Treg depletion experiment). In a coherent and complementary way, the addition of SANA in mixed lymphocyte reaction inhibited T-cell proliferation, a process that was also abrogated by the addition of the HO-1 inhibitor. The activation of HO-1 by SANA was roughly dependent on the intracellular TMEM176B cation channel (summarized in Figure 1). The TMEM176B channel is highly expressed in myeloid cells and has been previously associated with allograft tolerance.4 Accordingly, genetic deletion and blockade of TMEM176B enhance CD8-mediated antitumoral immune responses. In the present study, almost all of the SANA-mediated effects were abolished in TMEM176B–/– recipient mice, so the circle is complete. Finally, the combination of SANA with an anti-CD3 Fab antibody significantly prolonged the survival of minor mismatched skin allografts in wild-type recipients compared with the SANA condition alone. This underlines the synergy between these 2 substances, known to generate Treg. It should also be borne in mind that the administration of SANA stopped 15 d after transplantation, and it cannot be ruled out that prolonged administration may further delay the late rejection. Are these results of any interest to humans? The answer is yes because of the established role of Treg in human immunology as well as the numerous observations concerning the immunomodulatory and anti-inflammatory role of HO-1. In humans, the hmox1 promoter has a microsatellite polymorphism with (GT)n repeats, which is inversely correlated with HMOX1 transcriptional activity. Long repeat (low transcriptors) has been clearly associated with inflammatory diseases, including cardiovascular diseases, chronic kidney disease, HIV-induced central nervous system neuroinflammation, rheumatoid arthritis, organ transplantation outcome, and hematopoietic stem cell transplantation.5-11 Likely, an in-depth understanding of the multiple regulatory pathways involved in the allogeneic response and accurate mapping using powerful tools, such as artificial intelligence, will enable the development of new, personalized treatments that are more effective and less toxic than those currently available. This is at least one reason why it is important to be aware of the observation made by Galliussi et al.FIGURE 1.: Effect of the nitroalkene derivative of salicylate SANA on intragraft conventional MHCII-CD11ChighCD11b+ dendritic cells. SANA activates the TMEM176B cationic channel and the transcription factor Nrf2. Nrf2 binds the antioxidant response element (ARE) in chromatin, which results in HO-1 transcription. SANA also inhibits NF-κB and the NLRP3 inflammasome. Altogether, this will result in an increase of Treg infiltrate and reduced CD8+ T-cell effectors. ARE, antioxidant response element; HO-1, heme oxygenase-1; NF-κB, nuclear factor kappa B; NRLP3, nucleotide-binding domain, leucine-rich–containing family, pyrin domain–containing-3; SANA, salicylic acid nitroalkene; SNPP, tin protoporphyrin IX; Treg, regulatory T cell.