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.