Animal models to study mast cell function
Numerous types of animal models, primarily in the mouse, have been utilized throughout the years to outline the contribution of MCs in diverse pathological settings. In the first generation of such models, MC-deficiency in the mouse and rat was due to various mutations inKit , i.e. the receptor for SCF. Since SCF is an essential growth factor for MCs, defects in Kit result in an essentially complete absence of MCs. However, KIT is also expressed by a number of other cell types, and it has therefore been challenging to ascertain that consequences of Kit defects are indeed explained by an impact on the MC niche as opposed to off-target effects on other populations (reviewed in 107,108). To account for these issues, new mouse models of MC-deficiency, independent of Kit , have been developed (Table 2). These include mice where MC deficiency is driven by Cre recombinase expression under the control of MC-specific promoters. In one strategy, Cre recombinase was driven by the promoter forMcpt5 , a gene specifically expressed by CTMCs. These mice can then be crossed with R26DTA mice, leading to constitutive MC deficiency due to MC-specific expression of diphtheria toxin (DT) 109. Alternatively, the mice can be crossed with the iDTR line, leading to MC-specific expression of the DT receptor; treatment of these mice with DT will thus lead to conditional depletion of MCs 109. In another approach, MC deficiency was accomplished by inserting Cre under the control of the promoter for Cpa3, a gene expressed predominantly by MCs110. This leads to constitutive MC depletion, apparently due to Cre-mediated genotoxicity, but also to a substantial reduction of basophils, the latter in agreement with studies showing that basophils express low levels of Cpa3. The Cpa3 promoter was also exploited to generate a mouse strain in which the Cre-LoxP recombination system was used for deletion of the gene coding for the anti-apoptotic factor Mcl-1. This led to an essentially complete absence of MCs but also to a major reduction in basophils 111. MC depletion has also been accomplished in a model where the DT receptor gene was expressed under the control of a MC-specific IL-4 enhancer element 112. Another strategy was to insert the DT receptor and bright red td-Tomato fluorescent protein genes into the gene coding for the β chain of FcεRI, which is expressed by basophils and MCs. This approach can be used for depletion of MCs and basophils and also as an elegant tool to visualize MCs/basophils in vivo113. More recently, a mouse line with reduced numbers of MMCs (CTMCs were not affected) was generated by expressing Cre under the control of the baboon-α-chymase gene, and crossing these to mice with a floxed allele of Mcl-1 114.
By using these Kit -independent models of MC deficiency, important insight into the biological function of MCs has been obtained. As expected, the use of Kit -independent mouse models for MC-deficiency has firmly confirmed the essential role of MCs in allergic responses 110,115. Moreover, recent studies have shown that MCs can have a role in melanoma dissemination116, cutaneous lymphoma 117, collagen-induced arthritis 118, bone fracture-associated inflammation 119 and bone healing120. It was also demonstrated that MCs aggravate osteoarthritis 121 and can mediate the detrimental impact of smoke components on asthmatic features 122. Further, it has been demonstrated that MCs have a beneficial role in controlling bacterial clearance and promoting wound healing afterPseudomonas aeruginosa infection 123, whereas a detrimental impact of MCs was seen in skin infection by Sporothrix schenckii 124.
However, it is important to note that the use of these novel mouse models for MC deficiency has challenged some previous findings where a contribution of MCs in various pathologies has been implied. For example, recent findings based on Kit -independent mouse models for MC deficiency have questioned the role of MCs in certain models for autoimmune diseases 110 and obesity125, as well as the proposed adjuvant activities of MCs 118. Hence, a more nuanced view of how MCs are involved in pathological settings is currently emerging.
In addition to the various mouse models describe above, recent efforts have resulted in the generation of mice in which the MC niche is populated by human MCs. This was accomplished by transplanting human hematopoietic stem cells into NOD-scid IL2R-γ-/- mice, and then promoting MC growth by administrating plasmids expressing human SCF, GM-CSF and IL-3. In these mice (denoted “humice” or NSG-SGM3), mature human MCs (co-expressing KIT and Fcε RI) were detected in multiple tissues. These “humice” have so far mainly been used to study the role of MCs in anaphylaxis and cutaneous drug reactions126-128, but also in the evaluation of new therapies, e.g., using a BTK inhibitor 129. Moreover, functional human MCs could be developed from the bone marrow of these mice. Clearly, this humanized model has the potential to be used as a highly valuable tool to study human MC function.