Mast cells: versatile regulators of inflammation, tissue remodeling, host defense and homeostasis

Abstract

The possible roles of mast cells in heath and disease have been a topic of interest for over 125 years. Many adaptive or pathological processes affecting the skin or other anatomical sites have been associated with morphological evidence of mast cell activation, and/or with changes in mast cell numbers or phenotype. Such observations, taken together with the known functions of the diverse mediators, cytokines and growth factors which can be secreted by mast cells, have suggested many potential functions for mast cells in health and disease. Definitively identifying the importance of mast cells in biological responses in humans is difficult. However, mutant mice which are profoundly mast cell-deficient, especially those which can undergo engraftment with wild-type or genetically altered mast cells, provide an opportunity to investigate the importance of mast cells, and specific mast cell functions or products, in various adaptive or pathological responses in mice. Such work has shown that mast cells can significantly influence multiple features of inflammatory or immune responses, through diverse effects that can either promote or, surprisingly, suppress, aspects of these responses. Through such functions, mast cells can significantly influence inflammation, tissue remodeling, host defense and homeostasis.

Fig. 1

How to make mast cell knock-in mice, c-kit mutant genetically mast cell-deficient mice which have been selectively engrafted with genetically-compatible in vitro-derived mast cells. Mast cells are generated from bone marrow cells (or other hematopoietic cells, e.g., those in the fetal liver) from wild-type mice or from mice with specific genetic alterations. Alternatively, embryonic stem (ES) cell-derived cultured mast cells (ESCMCs) can be generated from wild-type or genetically-altered ES cells or mast cells can be transduced in vitro with shRNA to silence specific genes of interest. Those cultured mast cells can then be transplanted intra-venously (i.v.), intra-peritoneally (i.p.) or intra-dermally (i.d.) into mast cell-deficient c-kit mutant mice, such as WBB6F1-Kit^W/Kit^W-v or C57BL/6-Kit^W-sh/Kit^W-sh mice, to produce “mast cell knock-in mice”. A suitable interval is then allowed for the engraftment and phenotypic maturation of the adoptively transferred mast cells (the length of this interval can vary based on the route of mast cell transfer, the anatomical site of interest, etc.). The importance of mast cell function(s) in biological responses can be analyzed by comparing the responses in wild-type, c-kit mutant mast cell-deficient and selectively mast cell-engrafted c-kit mutant mice (mast cell knock-in mice). The contributions of specific mast cell products (surface structures, signaling molecules, secreted products, etc.) to such biological responses are analyzed by comparison of the features of the responses of interest in mast cell knock-in mice engrafted with wild-type mast cells versus mast cells derived from mice or ES cells that lack or express genetically-altered forms of such products or that have been transduced with shRNA to silence the specific genes that encode these products. Reproduced, with the permission of the publisher, from reference [4].

Fig. 2

Elongation of cutaneous nerves during CHS responses to oxazolone (OX) in C57BL/6 mice. (A) Tissue swelling responses to vehicle (open squares) or OX (closed squares) in C57BL/6 wild-type mice, which had been sensitized with 100 μl 2.0% OX in ethanol to shaved abdomen and challenged 5 days later with a total of 20 μl of ethanol to right (control) ear (10 μl to each side) and a total of 20 μl 1.0% OX in ethanol to left ear (10 μl to each side). Ear swelling was measured up to 48 h after challenge. (B) The length of PGP 9.5-positive or CGRP-positive nerve fibers in epidermis or dermis, respectively, in vehicle-treated ears (open columns) or OX-challenged ears (closed columns) of OX-sensitized mice 48 h after challenge. Data are the pooled results from three independent experiments, which gave similar results (vehicle: n = 5, OX; n = 13). In A, +++, ++++ = p < 0.001, < 0.0001 vs. corresponding values for vehicle-treated ears; In B, ++, +++ = p < 0.01, < 0.001 vs. corresponding values for vehicle-treated ears (Student’s t test, 2-tailed). Reproduced, with the permission of the publisher, from reference [48].

Fig. 3

Mast cell-derived TNF contributes to elongation of cutaneous nerves during CHS to OX in C57BL/6 mice. The length of (A) PGP 9.5-positive or (B) CGRP-positive nerve fibers in ears examined 48 h after hapten challenge in OX-sensitized C57BL/6 wild-type (vehicle; n=5, OX; n = 10), TNF^−/− mice (vehicle; n=5, OX; n = 10), mast cell-deficient Kit^W-sh/W-sh mice (vehicle; n=5, OX; n = 10), and Kit^W-sh/W-sh mice that had been selectively repaired of their mast cell deficiency by the intra-dermal injection of wild type or TNF^−/− BMCMCs (vehicle; n=5, OX; n = 9 in each group). Data show the length of nerve fibers at 48 h after OX or vehicle challenge in OX-sensitized mice. +, ++ = p < 0.05, < 0.01 vs. corresponding values for vehicle-treated ears; *, **, ***, **** = p < 0.05, < 0.01, < 0.001, < 0.0001 for the comparisons indicated by brackets (Student’s t test, 2-tailed). Reproduced, with the permission of the publisher, from reference [48].

Fig. 4

Mast cells and mast cell-derived IL-10 can limit the skin pathology associated with CHS to DNFB. Cross-sections of ears (stained with Masson’s Trichrome) from (A–C) WBB6F₁-Kit^+/+ (wild-type) mice, (D–F) mast cell-deficient Kit^W/W-v mice or Kit^W/W-v mice engrafted with wild-type (WT) BMCMCs (WT BMCMCs→Kit^W/W-v mice) (G–I) or Il10^−/− BMCMCs (Il10^−/− BMCMCs→Kit^W/W-v mice) (J–L) were obtained 5 d after challenge with vehicle (100% acetone) only (A, D, G and J) or 0.2% DNFB (B, E, H and K), or 10 d after challenge with 0.2% DNFB (C, F, I and L). Upper inset in E: arrowheads indicate border between intact epithelium (Ep) and, to the right, complete loss of epithelium at the base of an ulcer covered by an exudate. Ca*: cartilage; double-headed arrows show thickness of dermis (De) or epidermis (Ep); arrows in insets: sites of ulcers with adherent exudate (red). Scale bars in A and in insets in A & E = 100 μm. Photomicrographs are representative of the findings observed in the 6 different experiments we performed, with n = 4–6 mice/group per experiment; each experiment gave similar results. Reproduced, with the permission of the publisher, from reference [34].

Fig. 5

Mast cells are required for optimal resolution of tissue swelling after chronic low-dose UVB irradiation. Ear swelling examined for up to 22 d after the last of 15 exposures of 2 kJ/m² UVB irradiation in WBB6F₁-Kit^+/+ (wild-type [WT]) mice, mast cell-deficient Kit^W/W-v mice, or Kit^W/W-v mice engrafted with WT BMCMCs (WT BMCMCs→Kit^W/W-v mice) or Il10^−/− BMCMCs (Il10^−/−BMCMCs→Kit^W/W-v mice); n = 4–6 mice/group. The first exposure (arrow) was on day 0, immediately after we measured baseline ear thickness. Subsequent exposures are shown as arrowheads. ***P < 0.0001 versus corresponding UVB-irradiated wild-type mice; †††P < 0.0001 versus corresponding UVB-irradiated WT BMCMCs→Kit^W/W-v mice; ‡‡‡P < 0.0001 versus corresponding UVB-irradiated Il10^−/− BMCMCs→Kit^W/W-v mice. Reproduced, with the permission of the publisher, from reference [34].

Fig. 6

Mast cells can limit the toxicity of honeybee venom. (A) Changes in rectal temperature and (B) 24-hour survival and occurrence of gross hematuria after subcutaneous injection of A. mellifera venom (A.m.v.) at five different sites (three injections distributed over the length of the back skin and two into the belly skin, each containing 100 μg A.m.v. in 50 μl PBS) into WT mice, mast cell–deficient Kit^W-sh/Kit^W-sh mice, or Kit^W-sh/Kit^W-sh mice that had been engrafted intra-dermally, 6 weeks earlier, with 1.5 × 10⁶ BMCMCs into each of the five injection areas (WT BMCMCs→Kit^W-sh/Kit^W-sh). The amount of venom per injection (100 μg) roughly reflects the amount that can be delivered by one bee sting. All of the WT or WT BMCMCs→Kit^W-sh/Kit^W-sh mice appeared healthy, and their body temperatures returned to baseline within 2 days. ***P < 0.005 versus either C57BL/6 or WT BMCMCs→Kit^W-sh/Kit^W-sh mice. Reproduced, with the permission of the publisher, from reference [26].