There is overwhelming evidence linking IL-4 and IL-13 signaling to allergic asthma responses. Since IL-4 and IL-13 share receptor complexes, however, the exact contribution of the individual receptor complexes in inducing asthma pathophysiology is unclear. In this study, we used mice deficient in γc to elucidate the role of the Type I IL-4 receptor.
In addition to lacking the Type I IL-4 receptor, the γc−/−
mice are also deficient in IL-2R, IL-7R, IL-9R, IL-15R and IL-21R. IL-2 and IL-7 signaling play an important role in lymphocyte development and survival (reviewed in 
mice lack T and B cells. Therefore, we used RAG2−/−
mice as controls for all our experiments. Since TH
2 cells, and the cytokines they produce, are essential for initiation and propagation of allergic responses, we adoptively transferred in vivo
T cells from OT-II (OVA specific) transgenic mice. These transferred T cells express γc
and thus, receptors for IL-2, IL-4, IL-7 and IL-15. Therefore, they can respond to these cytokines even in a γc
deficient environment. We have demonstrated previously that TCR transgenic mice can be immunized with OVA/alum and that transfer of in vivo
T cells into mice followed by OVA/alum priming and OVA challenge was sufficient to induce features of allergic lung inflammation 
Mice lacking the Type II IL-4/IL-13 receptor (IL-13Rα1−/−
mice) still developed pulmonary inflammation and eosinophlia upon allergen challenge 
. Therefore, we hypothesized that IL-4 signals through the Type I R may be uniquely responsible for inducing these effects. However, our results show that the absence of the γc
chain caused no defect in these processes, suggesting that the Type I R is not absolutely required for mediating inflammatory responses and eosinophil recruitment into the lung. Since these responses are dependent on the IL-4Rα 
, the Type I and Type II receptors must mediate redundant functions for the inflammatory response.
Interestingly, we found that γc deficiency significantly enhanced lung pathology mediated by the transferred OT-II helper T-cells. While the exact mechanism involved is unknown, the increase in TH2 cytokines in the BAL fluid of γc−/− and γcxRAG2−/− mice, together with the reduction in IFNγ may contribute to this exaggerated asthma response. The increased amounts of IL-4 and IL-13 present in γc deficient mice could amplify signaling through the Type II R and enhance asthma responses.
We also observed that γc
deficiency in mice led to enhanced airway remodeling, leading to excessive collagen deposition and increase in smooth muscle thickness. IL-13 signaling through the Type II R is considered to be the dominant inducer of fibrosis. IL-13 induces macrophages to produce TGFβ and can act directly or indirectly on fibroblasts inducing collagen and extracellular matrix deposition (reviewed in 
). It has also been reported that both eosinophils and FIZZ1 and YM1 can cause lung fibrosis and smooth muscle thickening 
. In our model, the extent of inflammation in mice correlates well with the degree of airway remodeling.
Previous studies had indicated that gene expression of AAM products in the lung were differentially regulated by the Type I and Type II receptors: YM1 mRNA expression was partially dependent on IL-13Rα1, while FIZZ1 mRNA induction was completely independent of this chain 
. Here we show that epithelial cells in both RAG2−/−
mice were able to produce FIZZ1 and YM1, suggesting that induction of these proteins can occur independently of IL-4 signaling through the Type I R. We also observed that greater numbers of airways were YM1+ or FIZZ1+ in γc−/−
mice. This was surprising, as epithelial cells usually lack γc
expression. It is possible however, that the increased levels of IL-4 and IL-13 present in these mice results in greater engagement of these cytokines with the Type II R on epithelial cells, thus enhancing YM1 and FIZZ1 protein expression. Conversely, the decreased levels of IFNγ observed in the BAL fluid may be causing the enhanced TH
2 responses seen in epithelial cells. IFNγ signaling in airway epithelial cells has been reported to suppress STAT6 activation 
Our group had shown earlier that IL-4 induced robust AAM gene expression in BMM cells in vitro
while IL-13 was less potent in inducing the same responses 
. When macrophages lacked the γc
chain and the Type I R, however, their response to IL-4 was reduced, yet IL-13 responses were intact in these cells 
. Consistent with these in
studies, we found that YM1 protein expression in airway macrophages was reduced by half in the absence of the γc
chain, suggesting that the Type I R regulates YM1 protein expression in macrophages in vivo
The enhanced TH
2 responses and allergic lung inflammation occurring as a result of γc
deficiency was puzzling. In addition to the Type I IL-4R, the IL-9R, IL-15R and IL-21R are also absent in γc−/−
mice. Studies in IL-9 deficient mice have demonstrated that TH
2 differentiation, eosinophilic inflammation, AHR, mucus and IgE production occurred normally 
; IL-9 is mainly required for mast cell function and airway remodeling in chronic asthma 
. Importantly, these findings and other literature show a positive correlation between IL-9 and asthma pathogenesis. Thus, it is unlikely that the absence of IL-9 signaling in our model (which is mast cell-independent) is responsible for the enhanced allergic lung inflammation seen in γc−/−
mice. IL-21 is a γc
-dependent cytokine that is mainly produced by activated T cells and it is required for differentiation of TH
17 cells and T follicular helper (Tfh) cells (reviewed in 
). Since all the mice in our model received WT T cells, however, they would be able to respond to IL-21.
IL-15 regulates NK cell development, and loss of IL-15 or IL-15R results in a profound reduction in NK cell numbers 
. In confirmation of published studies, we found that the numbers of NK cells were reduced in mice lacking γc
. It has been reported that IL-4 induces IFNγ production in NK cells 
and IFNγ is known to suppress TH
2 responses. Thus, we examined whether reduced NK cell numbers was responsible for the asthma phenotype seen in our model. We found, however, that both depletion of NK cells in RAG2−/−
mice or transfer of NK cells into γc
mice did not alter features of allergic lung disease.
Recently, several groups have identified a novel population of innate lymphocytes called type 2 innate lymphoid cells (ILC2), that do not express standard lineage markers (reviewed in 
). These cells are found in the lung and exacerbate allergic inflammation by producing IL-13 and directly inducing AHR 
. However, they are dependent on the γc
chain and are absent in γc−/−
. Our data suggests that in the presence of TH
2 cells, these innate lymphocytes are not required.
It is possible that there is altered TH
2 priming in the γc
deficient environment. Since we provided a population of γc+/+
CD4+ OT-II T cells that have been primed only once in vivo
, we performed additional rounds of OVA/alum priming in the host after adoptive transfer. Dendritic cells play an integral role in T cell priming, and therefore, we postulated that the absence of γc
on DCs may cause dysregulated T cell priming. Indeed, we observed an increase in the TH
2 cytokine production and an increase in the percentage of Foxp3+
cells when T cells were primed and challenged with OVA in the γc
–deficient environment. It will be interesting to determine whether the Foxp3+
cells maintain suppressor function or acquire the ability to make effector cytokines. In addition to the T-cell changes, we found that the number of CD11b+
DCs was reduced in γc
mice while OX40L+
cells were modestly increased in these mice. DCs express both the Type I and Type II receptor and it has been established that these two receptors have differential roles in DC function. Lutz et. al.
demonstrated that both IL-4 and IL-13 promote DC maturation by signaling mainly through the Type II R 
. In contrast, the Type I R induces IL-12 production in DCs. It is conceivable that in γc
deficient mice, absence of this negative signaling loop causes enhanced TH
2 priming. OX40L expression in DCs is associated with increased TH
2 differentiation in absence of IL-12 
. It has also been reported that the balance between myeloid DCs and plasmacytoid DCs is altered in asthma, with a significant increase in the numbers of pDCs in asthma patients 
. The reduction in CD11b+ DCs in γc
mice points to a reduction in the numbers of mDCs in these mice, since CD11b is a marker for this subset of cells.
In summary, these results demonstrate that expression of γc is not required for eliciting effector asthma responses such as pulmonary inflammation, recruitment of eosinophils and mucus production. In the absence of the Type I R, the Type II R is sufficient to mediate these responses. In contrast, AAM protein expression in macrophages was dependent on the Type I R. Mice deficient in γc, however, developed a severe asthma phenotype when compared to control mice. Elevated TH2 cytokine production may be responsible for the exacerbated asthma responses seen in γc deficient mice.