Deleting Mcl-1 in mast cells: getting 2 birds with 1 stone

In this issue of Blood, Lilla and colleagues report that Mcl-1 (myeloid cell leukemia sequence 1), an intracellular antiapoptotic factor in a multiple hematopoietic lineage cells, plays a crucial role in regulating survival of both mast cells and basophils.¹ 

Mcl-1 belongs to the bcl-2 family proteins and has been considered a master regulator of cell survival. Earlier studies that used Mcl-1-floxed mice that express Cre recombinase under different cell type–specific promoters demonstrated that Mcl-1 plays an essential role in maintaining survival of multiple cell types including lymphocytes, neutrophils, hepatocytes, and neurons.^2-4  As both mast cells and basophils are also known to express Mcl-1, the authors directly investigated whether Mcl-1 exerts antiapoptotic functions in these cells by selectively deleting Mcl-1 by expressing Cre recombinase under the promoter of carboxypeptidase A3 (CPA3; referred to as Cpa3-Cre;Mcl-1^fl/fl mice hereafter), a gene highly expressed in mast cells and basophils. First, they confirmed CPA3-induced Cre recombinase expression in various cell types. As expected, high levels of Cre expression was observed in peritoneal mast cells. Conversely, the level of CPA3-induced Cre expression in basophils was substantially lower than that of mast cells. Moreover, the level of Cre expression in other granulocytes such as eosinophils and neutrophils was not different from that in basophils. When Cpa3-Cre; Mcl-1^fl/fl mice were analyzed, mast cell numbers were dramatically reduced in all tested tissues. The authors also found that the numbers of basophils in these mice were substantially diminished, while other granulocytes including eosinophils and neutrophils, despite the similar CPA3-induced Cre expression, were not affected by the lack of Mcl-1. Therefore, a selective defect in the survival of both mast cells and basophils is achieved in the Cpa3-Cre;Mcl-1^fl/fl mice.

Using the Cpa3-Cre;Mcl-1^fl/fl mice, the authors then tested if Mcl-1 deficiency in mast cells and basophils results in impaired biologic function of these cells in vivo. Consistent with reduced numbers, all tested inflammatory responses, mast cell–dependent passive cutaneous anaphylaxis, basophil-dependent chronic allergic inflammation, and IgE-dependent passive systemic anaphylaxis were severely impaired in Cpa3-Cre;Mcl-1^fl/fl mice, demonstrating that Mcl-1 is indeed indispensable for the development, survival, and biologic functions of both mast cells and basophils.

Particularly interesting from the current study is that the survival of mast cells and basophils in Cpa3-Cre;Mcl-1^fl/fl mice appears to be differentially regulated; for example, survival defect of mast cells is much greater than that of basophils. Although this could be attributed to the different levels of Cre expression in these cells, the finding that Mcl-1 deficiency fails to affect the homeostasis of other granulocytes that express similar CPA3-induced Cre strongly suggests that an additional survival factor might be involved. Measuring expression of Mcl-1 as well as other antiapoptotic molecules in different cell types of Cpa3-Cre;Mcl-1^fl/fl mice and whether the level of expression changes during immune responses will thus be of great importance. Likewise, LysM-Cre;Mcl-1^fl/fl mice in which Mcl-1 is selectively deleted in myeloid precursors result in severe neutropenia, although macrophage development normally occurs.² 

Elucidating a mechanism(s) underlying cell type–specific Mci-1–dependent (and independent) survival remains to be an area of future investigation. Specifically, a factor that induces Mcl-1 needs to be identified as Mcl-1 may be induced by different mechanism depending on the cell types. For example, IL-3 up-regulates Mcl-1 expression in basophils,⁵  although the lack of IL-3 has no obvious defect in maintaining basal level production of mast cells and basophils,⁶  strongly suggesting that Mcl-1–dependent survival of mast cells and basophils reported in the current study may be IL-3–independent. Alternatively, IL-5 and stem cell factor may be involved in up-regulating Mcl-1 as demonstrated in erythroleukemic cell line.⁷  Mcl-1 expression may also be modulated depending on the maturation status.⁸  In support of this possibility, molecular pathways leading to Mcl-1 expression were reported to be diverse.^5,7  Equally important is how Mcl-1 exerts antiapoptotic functions. Mcl-1 deficiency-induced apoptosis can be reversed by codeletion of proapoptotic molecules such as BIM, BAX, BAK, and PUMA,²  suggesting that a balance between antiapoptotic and proapoptotic pathways is needed. Mcl-1 may regulate expression of apoptosis mediators. Mcl-1 was also shown to be a direct target of caspases.⁹  In this context, investigating mechanisms underlying Mcl-1–mediated cell survival in different cell types will be important.

In sum, mast cells and basophils share similarity in phenotypes as well as in developmental pathways from which they arise, although recent studies have identified some nonredundant roles of each subset in immunity. Importantly, animal models developed thus far showed no common defects in either subset. Mast cell–deficient Kit^W/Wv or Kit^W-sh/W-sh mice carry mutations of the c-Kit gene, which obviously do not affect basophil biology. On the other hand, basophil-deficient Mcpt8-DTR or basoph8 mice carry a transgenic receptor that mediates toxin-mediated cell death and mast cells in these mice remain intact. In mice deficient in IL-3, both mast cell and basophil generation are severely impaired only after parasite infection.⁶  Therefore, Cpa3-Cre;Mcl-1^fl/fl mice will prove a useful animal model from which both redundant and nonredundant functions of mast cells and basophils in vivo can be unveiled.