Several studies have demonstrated roles for eosinophils during innate and adaptive immune responses to helminth infections. However, evidence that eosinophils are capable of initiating an immune response to parasite antigens is lacking. The goal of the present in vitro study was to investigate the potential of eosinophils to serve as antigen-presenting cells (APC) and initiate an immune response to parasite antigens. Purified eosinophils were exposed to soluble Strongyloides stercoralis antigens, and the expression of various surface markers involved in cell activation was examined. Antigen-exposed eosinophils showed a sixfold increase in expression levels of CD69 and major histocompatibility complex (MHC) class II, a fourfold increase in levels of T-cell costimulatory molecule CD86, and a twofold decrease in levels of CD62L compared to eosinophils cultured in medium containing granulocyte-macrophage colony-stimulating factor. The ability of eosinophils to present antigen to T cells was determined by culturing them with T cells in vitro. Eosinophils pulsed with antigen stimulated antigen-specific primed T cells and CD4 ؉ T cells to increase interleukin-5 (IL-5) production. The blocking of MHC class II expression on eosinophils inhibited their ability to induce IL-5 production by CD4؉ T cells in culture. Antigen-pulsed eosinophils were able to prime naïve T cells and CD4 ؉ T cells in culture and polarized them into Th2 cells producing IL-5 similar to that induced by antigen-loaded dendritic cells. These results demonstrate that eosinophils are capable of activating antigen-specific Th2 cells inducing the release of cytokines and assist in the priming of naïve T cells to initiate Th2 responses against infection. This study highlights the potential of eosinophils to actively induce immune responses against infection by amplifying antigen-specific Th2-cell responses.

The objective of the present study was to explore the ability of eosinophils to present Strongyloides stercoralis antigen in naive and immunized mice. Antigen-pulsed eosinophils were injected intraperitoneally into naive or immunized mice, and then mice were examined for antigen-specific immune responses. A single inoculation of antigen-pulsed eosinophils was sufficient to prime naive mice and to boost immunized mice for antigen-specific T helper cell type 2 (Th2) immune responses with increased interleukin (IL)-4 and IL-5 production. Mice inoculated 3 times with live eosinophils pulsed with antigen showed significant increases in parasite antigen-specific immunoglobulin (Ig) M and IgG levels in their serum. Antigen-pulsed eosinophils deficient in major histocompatibility complex class II molecules or antigen-pulsed dead eosinophils failed to induce immune responses, thereby demonstrating the requirement for direct interaction between eosinophils and T cells. These experiments demonstrate that eosinophils function as antigen-presenting cells for the induction of the primary and the expansion of the secondary Th2 immune responses to S. stercoralis in mice.

Recent studies have demonstrated the critical role of IL-10 in susceptibility to cutaneous and visceral leishmaniasis caused by Leishmania major and Leishmania donovani, respectively. To determine whether IL-10 also plays a similar role in the susceptibility and pathogenesis of cutaneous leishmaniasis caused by the New World species, L. mexicana and L. amazonensis, we analyzed their course of infection in IL-10-deficient BALB/c mice and their wild-type counterparts. Although IL-10-deficient mice infected with either L. mexicana or L. amazonensis failed to control the lesion progression, we did observe consistently lower levels of infection in IL-10−/− mice compared with wild-type BALB/c mice. We also observed increased IFN-γ and NO production and higher levels for IL-12p40 and IL-12Rβ2 mRNA in cells from IL-10−/− mice compared with cells from BALB/c mice. The mRNA levels for IL-4, which increased significantly in both IL-10−/− and BALB/c mice, were comparable. When treated with anti-IL-4 mAb, IL-10−/− mice resolved the infection more effectively and had significantly fewer parasites in their lesions compared with similarly treated BALB/c mice. These findings suggest that IL-10, although not the dominant mediator of susceptibility of BALB/c mice to infection with L. mexicana and L. amazonensis, does play a significant role in regulating the development of a protective Th1-type response. However, effective resolution of infection with these New World parasites requires neutralization of both IL-4 and IL-10.

Previous studies have shown that the in vitro ligation of FcγRs with IgG-opsonized Leishmania amastigotes promotes IL-10 production by macrophages. In addition, infection of either BALB/c mice lacking the common γ-chain of Fc receptors (FcγR−/−) or mice genetically altered to lack circulating Ab (JHD) with Leishmania pifanoi results in reduced and delayed lesion development and a deficit in the recruitment of inflammatory cells into infected lesions. We show in this study that FcγR−/− mice can control infection with Leishmania major and totally resolve cutaneous lesions. The ability to eventually control infection is not associated with a reduction in lesion inflammation or a reduction in the ability of Leishmania to parasitize cells through week 6 of infection. The immune response in healing FcγR−/− mice is associated with a reduction in numbers of cells producing Th2-type cytokines, including IL-4 and IL-10, but not an increase in numbers of IFN-γ-producing cells characteristic of a dominant Th1-type response. Instead, we observe a reduction in levels of IL-10 and TGF-β within infected lesions, including reduced levels of these cytokines within parasitized macrophages. Together, these results suggest that uptake of opsonized parasites via FcγRs may be a strong in vivo stimulus for the production of anti-inflammatory cytokines that play a role in susceptibility to infection.

CD40-CD40L interactions have been shown to be essential for the production of IL-12 and IFN-γ and control of L. major infection. In contrast, C57BL/6 mice deficient in CD28 develop a dominant Th1-type response and heal infection. In this study, we investigate the effects of a deficiency in both CD40L and CD28 molecules on the immune response and the course of L. major infection. We compared infection in mice genetically lacking CD40L (CD40L−/−), CD28 (CD28−/−), or both (CD40L−/−CD28−/−), and in C57BL/6 mice, all on a resistant background. Although CD40L−/− mice failed to control infection, CD28−/− and CD40L−/−CD28−/− mice, as well as C57BL/6 mice, spontaneously resolved their infections. Healing mice had reduced numbers of lesion parasites compared with nonhealing CD40L−/− mice. At wk 9 of infection, we detected similar levels of IL-4, IFN-γ, IL-12p40, and IL-12Rβ2 mRNA in draining lymph nodes of healing C57BL/6, CD28−/−, and CD40L−/−CD28−/− mice, whereas CD40L−/− mice had increased mRNA levels for IL-4 but reduced levels for IFN-γ, IL-12p40, and IL-12Rβ2. In a separate experiment, blocking of the CD40-CD40L pathway using Ab to CD40L led to an exacerbation of infection in C57BL/6 mice, but had little or no effect on infection in CD28−/− mice. Together, these results demonstrate that in the absence of CD28 costimulation, CD40-CD40L interaction is not required for the development of a protective Th1-type response. The expression of IL-12p40, IL-12Rβ2, and IFN-γ in CD40L−/−CD28−/− mice further suggests the presence of an additional stimulus capable of regulating IL-12 and its receptors in absence of CD40-CD40L interactions.