Studies in mice infected with the gastrointestinal nematode parasite Nippostrongylus brasiliensis demonstrated that IL-4/IL-13 activation of Stat6 suppresses development of intestinal mastocytosis and does not contribute to IL-4/IL-13 production, but is still essential for parasite expulsion. Because expulsion of another gastrointestinal nematode, Trichinella spiralis, unlike N. brasiliensis expulsion, is mast cell dependent, these observations suggested that T. spiralis expulsion would be Stat6 independent. Instead, we find that Stat6 activation by IL-4/IL-13 is required in T. spiralis-infected mice for the mast cell responses that induce worm expulsion and for the cytokine responses that induce intestinal mastocytosis. Furthermore, although IL-4 induces N. brasiliensis expulsion in the absence of B cells, T cells, and mast cells, mast cells and T cells are required for IL-4 induction of T. spiralis expulsion. Thus, Stat6 signaling is required for host protection against N. brasiliensis and T. spiralis but contributes to expulsion of these two worms by different mechanisms. The induction of multiple effector mechanisms by Stat6 signaling provides a way for a cytokine response induced by most gastrointestinal nematode parasites to protect against most of these parasites, even though different effector mechanisms are required for protection against different nematodes.

Although in vitro development of a Th2 response from naive CD4+ T cells is Stat6 dependent, mice immunized with a goat Ab to mouse IgD have been reported to produce a normal primary IL-4 response in Stat6-deficient mice. Experiments have now been performed with mice immunized with more conventional Ags or inoculated with nematode parasites to account for this apparent discrepancy. The ability of an immunogen to induce a primary in vivo IL-4 response in Stat6-deficient mice was found to vary directly with its ability to induce a strong type 2 cytokine-biased response in normal mice. Even immunogens, however, that induce strong primary IL-4 responses in Stat6-deficient mice induce poor memory IL-4 responses in these mice. Consistent with this, Stat6-deficient CD4+ T cells make relatively normal IL-4 responses when stimulated in vitro for 3 days with anti-CD3 and anti-CD28, but poor IL-4 responses if they are later restimulated with anti-CD3. Thus, Stat6 signaling enhances primary IL-4 responses that are made as part of a type 0 cytokine response (mixed type 1 and type 2) and is required for normal development or survival of Th2 memory cells.

Experiments were performed to characterize and identify the cellular sources of the secondary interleukin (IL)-4 response to a T cell–dependent antigen. Mice were primed by immunization with goat anti–mouse immunoglobulin (Ig)D antibody (GaMD), which stimulates naive CD4+ T cells to secrete IL-4 in 3–4 d. When challenged with goat serum 14 d after immunization, GaMD-primed mice generated an IL-4 response that exceeded the primary response by ∼100-fold, started in <2 h, and lasted for 4 d. Studies with 4get mice, in which cells with an accessible Il4 gene express a green fluorescent protein (GFP), revealed CD4+ memory T cells, natural killer T cells, basophils, mast cells, and eosinophils as possible rapid producers of IL-4. GFP+CD4+ T cells and basophils expanded more in the spleen than the other cell types during the primary response to GaMD. Quantitation of in vivo IL-4 production by the in vivo cytokine capture assay after individual cell types were selectively stimulated or deleted demonstrated that basophils and memory CD4+ T cells account for most of the secondary IL-4 response, with basophils initiating that response through IgE/FcɛRI-mediated signaling but secreting IL-4 for <4 h and memory T cells secreting IL-4 within 4 h and continuing to secrete this cytokine for 4 d.

Although IL-4 and IFN-γ often have opposite effects and suppress each other’s production by T cells, IL-4 can stimulate IFN-γ production. To characterize this, we injected mice with IL-4 and quantified IFN-γ production with the in vivo cytokine capture assay. IL-4 induced Stat6-dependent IFN-γ production by NK and, to a lesser extent, NKT cells, but not conventional T cells, in 2–4 h. Increased IFN-γ production persisted at a constant rate for &gt;24 h, but eventually declined, even with continuing IL-4 stimulation. This eventual decline in IFN-γ production was accompanied by a decrease in NK and T cell numbers. Consistent with a dominant role for NK cells in IL-4-stimulated IFN-γ secretion, IL-4 induction of IFN-γ was B and T cell-independent; suppressed by an anti-IL-2Rβ mAb that eliminates most NK and NKT cells; reduced in Stat4-deficient mice, which have decreased numbers of NK cells; and absent in Rag2/γc-double-deficient mice, which lack T, B, and NK cells. IL-4-induced IFN-γ production was not affected by neutralizing IL-12p40 and was increased by neutralizing IL-2. IL-13, which signals through the type 2 IL-4R and mimics many IL-4 effects, failed to stimulate IFN-γ production and, in most experiments, suppressed basal IFN-γ production. Thus, IL-4, acting through the type 1 IL-4R, induces Stat6-dependent IFN-γ secretion by NK and NKT cells. This explains how IL-4 can contribute to Th1 cytokine-associated immune effector functions and suggests how IL-13 can have stronger proallergic effects than IL-4.

Because most cytokines are utilized, catabolized, or excreted shortly after they are produced, it has been difficult to directly measure in vivo cytokine production. Consequently, it has been necessary to infer in vivo cytokine secretion levels from the results of ex vivo assays of cytokine secretion, assays that measure tissue levels of cytokine mRNA, or assays that stain tissues for cytokine protein levels. Results of these assays provide important and useful information, but do not necessarily reflect in vivo cytokine secretion. To better determine in vivo cytokine production, the in vivo cytokine capture assay (IVCCA) was developed. IVCCA facilitates measurement of cytokines in serum by increasing their in vivo half-lives. This increases the sensitivity of measurement of in vivo cytokine production 30- to 1,000-fold. The first protocol described in this unit is for luminescence-based ELISA, while the second is for an absorbance-based method.