H-Leu-Leu-OMe . HBr

H-Leu-Leu-OMe . HBr (Leu-Leu methyl ester hydrobromide) is a dipeptide generated by human monocytes or polymorphonuclear leukocytes. H-Leu-Leu-OMe . HBr selectively eliminates lymphocytes, especially natural killer cells with cytotoxic potential, and induces endolysosomal pathway stress^[1]. The sensitivity of cytotoxic lymphocytes to H-Leu-Leu-OMe . HBr depends upon the production of membranolytic metabolites of the structure (Leu-Leu)n-OMe by dipeptidyl peptidase I^[2]. Such membranolytic metabolites are present at far higher levels in cytotoxic lymphocytes than in cells without cytolytic potential or not of bone marrow origin^[2].

In vitro, H-Leu-Leu-OMe . HBr may act as an inflammasome activator on RPE cells (primary human fetal RPE cells; 1mM; 3h), leading to caspase-1 cleavage, cytokine secretion, and X-linked inhibitor of apoptosis protein (XIAP) reduction^[3]. H-Leu-Leu-OMe . HBr activates NLRP3 inflammasome that promotes IL-6 and IL-8 production (human umbilical vein endothelial cells, 20μM; 19h) in primed human umbilical vein endothelial cells^[4]. Moreover, H-Leu-Leu-OMe . HBr may act as an apoptosis inducer (U937 cells; 0.5mM; 30min) in natural killer cells and macrophages^[5].

In vivo, H-Leu-Leu-OMe . HBr shows toxicity to lymphocytes (adult (CBA x BALB/c) F₁ mice, H-Leu-Leu-OMe . HBr-treated CBA spleen cells(0.15M; 15min; room temperature; i.p) that suppressing lymphocyte function^[6]. H-Leu-Leu-OMe . HBr can also mimic alum-mediated necrosis that specifically kills myeloid leukocytes in 8-week-old female C57BL/6 mice (1mg; once; i.v)^[7].

References:
[1] Thiele, D L, and P E Lipsky. “The immunosuppressive activity of L-leucyl-L-leucine methyl ester: selective ablation of cytotoxic lymphocytes and monocytes.” *Journal of immunology (Baltimore, Md. : 1950)* vol. 136,3 (1986): 1038-48.
[2] Thiele, D L, and P E Lipsky. “Mechanism of L-leucyl-L-leucine methyl ester-mediated killing of cytotoxic lymphocytes: dependence on a lysosomal thiol protease, dipeptidyl peptidase I, that is enriched in these cells.” *Proceedings of the National Academy of Sciences of the United States of America* vol. 87,1 (1990): 83-7. doi:10.1073/pnas.87.1.83
[3] Gao, Jiangyuan et al. “The reduction of XIAP is associated with inflammasome activation in RPE: implications for AMD pathogenesis.” Journal of neuroinflammation vol. 16,1 171. 22 Aug. 2019, doi:10.1186/s12974-019-1558-5
[4] Kinnunen, K et al. “Lysosomal destabilization activates the NLRP3 inflammasome in human umbilical vein endothelial cells (HUVECs).” *Journal of cell communication and signaling* vol. 11,3 (2017): 275-279. doi:10.1007/s12079-017-0396-4
[5] Odaka, M et al. “Synthesis, photoreactivity and cytotoxic activity of caged compounds of L-leucyl-L-leucine methyl ester, an apoptosis inducer.” *Photochemistry and photobiology* vol. 63,6 (1996): 800-6. doi:10.1111/j.1751-1097.1996.tb09633.x
[6] Mowat, A M, and P A Leck. “Generalized toxicity of L-leucyl-leucine-methyl ester for lymphocyte functions.” *Immunology* vol. 69,4 (1990): 564-9.
[7]Jacobson, Lee S et al. “Cathepsin-mediated necrosis controls the adaptive immune response by Th2 (T helper type 2)-associated adjuvants.” *The Journal of biological chemistry* vol. 288,11 (2013): 7481-7491. doi:10.1074/jbc.M112.400655