Basophil granulocytes and mast cells are recognized for their roles in immunity and are central effectors of diverse immunological disorders. Despite their similarities, there is emerging evidence for non-redundant roles of the circulating yet scarce basophils and tissue-resident mast cells, respectively. Because of their importance in allergic pathogenesis, specific induction of apoptosis in basophils and mast cells may represent an interesting novel treatment strategy. The pro-inflammatory cytokine interleukin-3 serves as a key factor for basophil and mouse mast cell survival. Interleukin-3 increases the expression of anti-apoptotic BCL-2 family members, such as BCL-2, BCL-XL or MCL-1; however, little is known how strongly these individual proteins contribute to basophil survival. Here, we were applying small molecule inhibitors called BH3 mimetics, some of which show remarkable success in cancer treatments, to neutralize the function of anti-apoptotic BCL-2 family members. We observed that expression levels of anti-apoptotic BCL-2 proteins do not necessarily correlate with their respective importance for basophil survival. Whereas naive in vitro-differentiated mouse basophils efficiently died upon BCL-2 or BCL-XL inhibition, interleukin-3 priming rendered the cells highly resistant toward apoptosis, and this could only be overcome upon combined targeting of BCL-2 and BCL-XL. Of note, human basophils differed from mouse basophils as they depended on BCL-2 and MCL-1, but not on BCL-XL, for their survival at steady state. On the other hand, and in contrast to mouse basophils, MCL-1 proved critical in mediating survival of interleukin-3 stimulated mouse mast cells, whereas BCL-XL seemed dispensable. Taken together, our results indicate that by choosing the right combination of BH3 mimetic compounds, basophils and mast cells can be efficiently killed, even after stimulation with potent pro-survival cytokines such as interleukin-3. Because of the tolerable side effects of BH3 mimetics, targeting basophils or mast cells for apoptosis opens interesting possibilities for novel treatment approaches.

BH3-mimetics are small molecule inhibitors that neutralize the function of anti-apoptotic BCL-2 family members. BH3-mimetics have recently gained a lot of popularity in oncology because of their success in cancer treatment. However, BH3-mimetics might have a broader clinical application. Here, we established an ex vivo flow cytometric assay allowing the comparison of the impact of BH3-mimetics (ABT-199, ABT-263, WEHI-539, and S63845) on leukocyte populations of both, healthy human subjects and C57BL/6 J wild type mice. BH3-mimetics were added to freshly drawn blood that was diluted 1/2 in cell medium, and BH3-mimetics-mediated impact on leukocyte count was assessed by flow cytometry. Our results demonstrate that responses towards 1μM of BH3-mimetics can be identical as well as considerably different in leukocytes of humans and mice. For instance, the inhibition of BCL-2 by ABT-199 caused cell death in all types of lymphocytes in mice but was exclusively specific for B cells in humans. Moreover, inhibition of BCL-X L by WEHI-539 affected solely mouse leukocytes while targeting MCL-1 by S63845 resulted in efficient induction of cell death in human neutrophils but not in their mouse counterparts. Our ex vivo assay enables initial identification of analogies and differences between human and mouse leukocytes in response towards BH3-mimetics. Excessive cell death or evasion from apoptosis is associated with the development of autoimmune disorders or cancer 1. Apoptosis is primarily regulated by B cell lymphoma 2 (BCL-2) family members, which differ based on their ability to promote or to suppress apoptosis by affecting the integrity of the mitochondrial outer membrane (MOM). Anti-apoptotic BCL-2 family members (BCL-2, BCL-X L , BCL-W, MCL-1, BFL-1/A1) function by inhibiting their pro-apoptotic counterparts, which, upon their "unleashing", trigger MOM permeabilization followed by activation of apoptotic caspases, culminating in the ultimate, irreversible cascade towards cellular demise 2,3. Previous studies have shown that the interplay of pro-and anti-apoptotic BCL-2 members specifically regulates hematopoiesis and survival of leukocytes in blood and bone marrow 4,5. Importantly, upregulation of certain anti-apoptotic BCL-2 family members, such as BCL-2, B cell lymphoma-extra-large (BCL-X L), and myeloid cell leukemia (MCL-1), are also known to support development and survival of cancerous cells as well as evasion from cancer therapy 6. To overcome such resistance towards apoptosis, small molecule inhibitors called BCL-2 homology domain 3 (BH3)-mimetics have been developed to specifically neutralize anti-apoptotic BCL-2 family members in order to restore normal apoptotic signaling in cancerous cells 7. Most studies involving BH3-mimetics have been carried out in cancer cells. However, with some exceptions, the impact of these compounds on key leukocyte populations from healthy individuals is less clear. As the expression of individual anti-apoptotic BCL-2 family members varies substantially between different leukocyte...

our results, and further proteomics and functional studies are needed to elucidate the role these genes play in relation to changes in Staphylococcus species and skin barrier dysfunction in general.As a general conclusion, we postulate that the explanation for AD flare development and exacerbation is a complex interaction between the skin microbiota, barrier, and immune system. Interaction points between different components of human skin might be the key to development of therapeutic approaches to allergic skin diseases. Guttman-Yassky E, et al. Cellular and molecular immunologic mechanisms in patients with atopic dermatitis. J Allergy Clin Immunol 2016;138:336-49. 4. Czarnowicki T, Krueger JG, Guttman-Yassky E. Novel concepts of prevention and treatment of atopic dermatitis through barrier and immune manipulations with implications for the atopic march. J Allergy Clin Immunol 2017;139: 1723-34. 5. Gong JQ, Lin L, Lin T, Hao F, Zeng FQ, Bi ZG, et al. Skin colonization by Staphylococcus aureus in patients with eczema and atopic dermatitis and relevant combined topical therapy: a double-blind multicentre randomized controlled trial. Br J Dermatol 2006;155:680-7. 6. Esaki H, Ewald DA, Ungar B, Rozenblit M, Zheng X, Xu H, et al. Identification of novel immune and barrier genes in atopic dermatitis by means of laser capture microdissection. J Allergy Clin Immunol 2015;135:153-63. 7. Williams MR, Gallo RL. The role of the skin microbiome in atopic dermatitis. Curr Allergy Asthma Rep 2015;15:65. 8. Suarez-Farinas M, Ungar B, Correa da Rosa J, Ewald DA, Rozenblit M, Gonzalez J, et al. RNA sequencing atopic dermatitis transcriptome profiling provides insights into novel disease mechanisms with potential therapeutic implications. J Allergy Clin Immunol 2015;135:1218-27. 9. Clark RT, Hope A, Lopez-Fraga M, Schiller N, Lo DD. Bacterial particle endocytosis by epithelial cells is selective and enhanced by tumor necrosis factor receptor ligands.