129S5 mice demonstrated increased susceptibility to both passive and active IgE-mediated anaphylaxis. The increased susceptibility was linked with elevated homeostatic tissue mast cells and increased sensitivity to histamine infusion. In vitro analysis demonstrated 129S5-derived BMMCs have increased proliferative capacity and decreased apoptosis. Collectively, these studies demonstrate that 129S5 mice have inherent differences in bone marrow-derived mast cell functional capacity and sensitivity to mast cell-derived mediators, and that these factors are sufficient to alter susceptibility to IgE mediated anaphylactic reactions.
Systemic and oral antigen-induced IgE-mediated anaphylaxis in mice is dependent on the IgE-FcεRI-mast cell pathway. Ag-IgE crosslinking of the FcεRI on mast cells leads to degranulation and release of preformed mediators, such as histamine, leading to anaphylactic shock [6
]. Indeed we demonstrated that antagonism of histamine activity blocked the anaphylactic response. Notably, 129S5 mice have increased susceptibility to IgE-mediated anaphylaxis that is linked with elevated systemic mast cells levels. We suggest that activation of a greater number of mast cells leads to increased systemic levels of mast cell mediators, such as histamine, and increase severity of the anaphylactic reaction. Consistent with this, we demonstrated increased serum histamine in 129S5 mice compared to BALB/c. Elevated serum histamine can also be attributed to increased degranulation efficiency of mast cells, but we found no difference in degranulation efficiency (i.e. equivalent hexosaminidase and cytokine release) of BMMCs from 129S5 and BALB/c mice suggesting that mast cell load may explain the observed differences between strains. Consistent with this, previous experimental studies employing IL-9 transgenic mice have demonstrated that elevated systemic mast cell levels increased IgE-induced mast cell mediator release and severity of anaphylaxis [34
Analyses of BMMC from 129S5 and BALB/c mice revealed differences in mast cell function. Moreover, 129S5 BMMC possessed increased proliferative capacity and reduced apoptosis. The increased proliferative capacity and survival of 129S5 BMMCs could explain the increased systemic mast cell levels in 129S5 mice compared with BALB/c.
We demonstrated that IgE-mediated anaphylaxis is histamine-dependent. Furthermore, we showed increased plasma histamine in 129S5 mice compared with BALB/c mice. Histamine regulates body temperature by acting on hypothalamic histaminergic neurons in the central nervous system [29
] or alternatively, and more relevant to a rapid anaphylactic reaction, via induction of vascular leak through dysregulation of endothelial gap formation of the vascular wall [35
]. Interestingly, 129S5 mice displayed increased susceptibility to intravenous histamine-induced hypothermia, suggesting a strain-dependent difference in vascular permeability. This is consistent with the previously reported strain-dependent differences in pulmonary vascular permeability in response to ischemia [37
]. Collectively, our data indicate that strain-dependent differences in tissue mast cell levels attributed to possible increased proliferation and apoptosis rates and in part an increase in histamine sensitivity heighten 129S5 mice to IgE-mast cell-mediated reactions.
Previous investigations have demonstrated that expression levels of the FcγRII can influence IgE-mast cell regulated anaphylaxis [38
]. Moreover, mice deficient in the FcγRIIB display enhanced IgE-mediated anaphylactic responses [38
]. Notably, the 129/Sv and 129/Ola strains of mice share a promoter haplotype associated with reduced expression and function of the FcγRII [40
]. To determine if the observed differences in severity of anaphylaxis between 129S5 and BALB/c is due to altered FcγR expression, we assessed FcγRII/III expression on peritoneal mast cells [39
]. We observed no significant difference in expression of FcγRII/III on 129S5 peritoneal mast cells (c-kit+
) compared to BALB/c (FcγRII/III mean fluoresence intensity [MFI] 61.9 ± 9.2 vs 74.4 ± 14.1; mean ±SD; n = 4 per group; not significant) suggesting that the level of FcγRII/III on mast cells is not the primary reason for the observed differences in IgE-mast cell-triggered reactions. Importantly, Takai et al., [31
] demonstrated that loss of FcγRII expression was associated with increased IgE-and IgG-triggered anaphylaxis. We did not observe any significant differences in the severity of IgG-mediated anaphylaxis between BALB/c and 129S5 mice further supporting the notion that increased severity of anaphylaxis in 129S5 mice is independent of FcγRII expression.
Previous experimental data has identified strain-dependent differences in mast cell function. Lyn-/-
mice on the C57BL6 and BALB/c background demonstrated opposite phenotypes with respect to mast cell degranulation efficiency [9
]. Furthermore, BALB/c BMMC-derived PGD synthase, IL-6 and MCP-1 levels are significantly elevated in comparison to C57BL6 [10
]. We are unable to assess differences in vivo anaphylactic phenotypes between 129S5, BALB/c and C57BL/6, as C57BL/6 mice are resistant to oral antigen-induced anaphylaxis induced by OVA sensitization and repeated oral challenge (results not shown). The fact that C57BL/6 mice (H-2b haplotype) are resistant to oral antigen-induced anaphylaxis, and the 129S5 (H-2b haplotype) have increased responsiveness compared to BALB/c (H-2d haplotype) indicates that severity of anaphylaxis in these strains is independent of MHC haplotypes. Furthermore, there are approximately 17 sub-strains in the 129 strain, which possess quite different genetic and phenotypic characteristics [41
]. Thus, it is possible that there are also differences in anaphylaxis phenotypes between 129 substrains that are yet to be identified. Our identification of substantial strain-dependent differences between the mast cells of 129S5 and BALB/c mice has important implications for the study of anaphylaxis and mast cell biology given that embryonic stem cells from 129S5 mice are widely used in experimental gene targeting methodology [42
]. The strain dependent differences indicate that rigorously designed breeding strategies for control of genetic background must be employed in such gene targeting studies to reduce the influence of strain dependent traits on IgE-mast cell responses in the knock out models.