Jude E. Uzonna

Central memory T cells mediate long-term immunity to Leishmania major in the absence of persistent parasites

Infection with Leishmania major induces a protective immune response and long-term resistance to reinfection, which is thought to depend upon persistent parasites. Here we demonstrate that although effector CD4+ T cells are lost in the absence of parasites, central memory CD4+ T cells are maintained. Upon secondary infection, these central memory T cells become tissue-homing effector T cells and mediate protection. Thus, immunity to L. major is mediated by at least two distinct populations of CD4+ T cells: short-lived pathogen-dependent effector cells and long-lived pathogen-independent central memory cells. These data suggest that central memory T cells should be the targets for nonlive vaccines against infectious diseases requiring cell-mediated immunity.

Low Dose Leishmania major Promotes a Transient T Helper Cell Type 2 Response That Is Down-regulated by Interferon γ–producing CD8+ T Cells

An unresolved issue in the field of T helper (Th) cell development relates to the findings that low doses of antigen promote Th2 cell development in vitro, whereas several classic in vivo studies suggest the opposite. Here we resolve this paradox by studying the early immune response in mice after infection with different doses of Leishmania major. We found that low parasite doses induced a Th2 response in C57BL/6 (B6) mice, whereas high doses induced a Th1 response. However, the Th2 response in low dose–infected mice was transient and the animals healed. The appearance of a Th1 response after low dose infection was dependent upon the concomitant activation of interferon γ–producing CD8+ T cells. In the absence of CD8+ T cells, the Th2 response was maintained. However, either neutralization of interleukin (IL)-4 or administration of IL-12 promoted a Th1 response after low dose infection of CD8-deficient mice, indicating that the required role for CD8+ T cells was limited to modulation of CD4+ T cell responses. Thus, the discrepant results seen between in vivo and in vitro studies on the effects of antigen dose on Th cell differentiation may depend upon whether CD8+ T cells participate in the immune response.

Vaccination with Phosphoglycan-Deficient Leishmania major Protects Highly Susceptible Mice from Virulent Challenge without Inducing a Strong Th1 Response

Long-term immunity to Leishmania may require the continued presence of parasites, but previous attempts to create attenuated parasites that persist without causing disease have had limited success. Since Leishmania major mutants that lack lipophosphoglycan and other secreted phosphoglycans, termed lpg2−, persist indefinitely in infected mice without inducing any disease, we tested their ability to provide protection to virulent L. major challenge. In response to leishmanial Ag stimulation, cells from lpg2−-infected mice produced minimal levels of IL-4 and IL-10, as well as very low levels of IFN-γ. Nevertheless, when BALB/c mice infected with lpg2− parasites were challenged with virulent L. major they were protected from disease. Thus, these findings report on attenuated parasites that may be used to induce long-term protection against leishmaniasis and indicate that the immunity induced can be maintained in the absence of a strong Th1 response.

The development of effector and memory T cells in cutaneous leishmaniasis: the implications for vaccine development

Summary:  Leishmania major infections induce the development of a CD4+ T‐helper 1 (Th1) response that not only controls the primary infection but also results in life‐long immunity to reinfection. How that immunity is maintained is unknown, although because of the existence of infection‐induced immunity, there has been an assumption that the development of a vaccine against leishmaniasis would be relatively easy. This has turned out not to be the case. One problem has been the finding that a large part of the immunity induced by a primary infection depends upon the presence of persistent parasites. Nevertheless, there are ample situations where immunologic memory persists without the continued presence of antigen, providing the prospect that a non‐live vaccine for leishmaniasis can be developed. To do so will require an understanding of the events involved in the development of an effective protective T‐cell response and, more importantly, an understanding of how to maintain that response. Here, we review work from our laboratory, describing how Th1 cells develop in L. major‐infected mice, the nature of the memory T cells that provide protection to reinfection, and how that information may be utilized in the development of vaccines.

The early interaction of Leishmania with macrophages and dendritic cells and its influence on the host immune response

The complicated interactions between Leishmania and the host antigen-presenting cells (APCs) have fundamental effects on the final outcome of the disease. Two major APCs, macrophages and dendritic cells (DCs), play critical roles in mediating resistance and susceptibility during Leishmania infection. Macrophages are the primary resident cell for Leishmania: they phagocytose and permit parasite proliferation. However, these cells are also the major effector cells to eliminate infection. The effective clearance of parasites by macrophages depends on activation of appropriate immune response, which is usually initiated by DCs. Here, we review the early interaction of APCs with Leishmania parasites and how these interactions profoundly impact on the ensuing adaptive immune response. We also discuss how the current knowledge will allow further refinement of our understanding of the interplay between Leishmania and its hosts that leads to resistance or susceptibility.

The p110δ Isoform of Phosphatidylinositol 3-Kinase Controls Susceptibility to Leishmania major by Regulating Expansion and Tissue Homing of Regulatory T Cells

Resistance to Leishmania major and most intracellular pathogens is usually associated with a strong T cell-mediated immunity, particularly a CD4+ Th1 response. Mice with an inactivating knock-in mutation in the p110δ isoform of PI3K (referred to as p110δD910A) show severely impaired T cell responses. Because a strong T cell response is thought to mediate resistance to intracellular pathogens, we examined the outcome of L. major infection in p110δD910A mice. Paradoxically, p110δD910A mice on “resistant” and “susceptible” genetic backgrounds showed more robust resistance manifested as significantly reduced lesion size and accelerated parasite clearance. This enhanced resistance was associated with dramatically diminished immune responses, including impaired cell proliferation and effector cytokine (IFN-γ and TNF) production. Interestingly, the ability of macrophages and dendritic cells from p110δD910A mice to produce NO and destroy Leishmania parasites was similar to those of wild-type mice. We show that the enhanced resistance of p110δD910A mice was due to impaired expansion and effector functions of regulatory T cells (Tregs). Adoptive transfer studies demonstrated that p110δD910A mice lost their increased resistance when given enriched Tregs from wild-type mice. We suggest on the basis of these and further observations that the lack of this enzyme prominently affects Treg expansion and homing to infection sites, and that in the absence of Tregs, weak Th1 responses are capable of containing parasites and prevent pathology. We also suggest that temporary pharmacological inhibition of this enzyme may be a very effective form of treatment against cutaneous leishmaniasis.

Efficacy of commercial and field-strain Mycobacterium paratuberculosis vaccinations with recombinant IL-12 in a bovine experimental infection model

The efficacy of commercial (Strain 18) and field-isolate paratuberculosis vaccine preparations was investigated. The effect of prior exposure to Mycobacterium paratuberculosis and the adjuvant effect of rIL-12 on vaccine efficacy were also tested. Both Strain 18 and field-isolate vaccines induced strong local, systemic and enteric IFN-gamma responses. A significant reduction in mycobacterial colonization was observed when calves were vaccinated with the field-isolate prior to challenge, but not following vaccination with Strain 18 vaccine. Vaccination with rIL-12 prevented infection in some calves but its overall effect on IFN-gamma response and total mycobacterial load was not statistically significant. Efficacy of paratuberculosis vaccines may be enhanced if calves are vaccinated prior to M. paratuberculosis exposure with field-isolate vaccine instead of Strain 18 vaccine currently in use.

Control of New World cutaneous leishmaniasis is IL‐12 independent but STAT4 dependent

Leishmania mexicana, a New World protozoan parasite, induces small, chronic, but non‐progressive lesions in C57BL/6 (B6) mice. In this study we investigated the role of IL‐12, and subsequent Th1 factors, in controlling cutaneous L. mexicana infection. IL‐12 treatment failed to promote disease resolution, suggesting that the inability of mice to heal is not related to a deficiency of endogenous IL‐12 production. Surprisingly, L. mexicana‐induced cutaneous lesions in wild‐type and IL‐12p40‐deficient mice were indistinguishable, with similar parasite burdens, immune responses, and lesion histopathology. In contrast, iNOS, IFN‐γ, and STAT4‐deficient mice developed progressive disease and uncontrolled parasite growth. These results differ dramatically from L. major infection, in which IL‐12p40‐deficient mice are highly susceptible, with very rapid lesion growth, very large parasite burdens, and the development of a strong Th2 response.These data uncover the existence of an alternate IFN‐γ and iNOS pathway for control of Leishmania lesions, which is IL‐12 independent, but which unexpectedly requires STAT4.

Infection with arginase deficient Leishmania major reveals a parasite number-dependent and cytokine-independent regulation of host cellular arginase activity and disease pathogenesis

The balance between the products of l-arginine metabolism in macrophages regulates the outcome of Leishmania major infection. l-arginine can be oxidized by host inducible NO synthase to produce NO, which contributes to parasite killing. In contrast, l-arginine hydrolysis by host arginase blocks NO generation and provides polyamines, which can support parasite proliferation. Additionally, Leishmania encode their own arginase which has considerable potential to modulate infectivity and disease pathogenesis. In this study, we compared the infectivity and impact on host cellular immune response in vitro and in vivo of wild-type (WT) L. major with that of a parasite arginase null mutant (arg−) L. major. We found that arg− L. major are impaired in their macrophage infectivity in vitro independent of host inducible NO synthase activities. As with in vitro results, the proliferation of arg− L. major in animal infections was also significantly impaired in vivo, resulting in delayed onset of lesion development, attenuated pathology, and low parasite burden. Despite this attenuated pathology, the production of cytokines by cells from the draining lymph node of mice infected with WT and arg− L. major was similar at all times tested. Interestingly, in vitro and in vivo arginase levels were significantly lower in arg− than in WT-infected cases and were directly correlated with parasite numbers inside infected cells. These results suggest that Leishmania-encoded arginase enhances disease pathogenesis by augmenting host cellular arginase activities and that contrary to previous in vitro studies, the host cytokine response does not influence host arginase activity.

Vaccines and vaccination strategies against human cutaneous leishmaniasis

One might think that the development of a vaccine against cutaneous leishmaniasis would be relatively straightforward because the type of immune response required for protection is known and natural immunity occurs following recovery from primary infection. However, there is as yet no effective vaccine against the disease in humans. Although vaccination in murine studies has yielded promising results, these vaccines have failed miserably when tested in primates or humans. The reasons behind these failures are unknown and remain a major hurdle for vaccine design and development against cutaneous leishmaniasis. In contrast, recovery from natural, deliberate or experimental infections results in development of long-lasting immunity to re-infection. This so called infection-induced resistance is the strongest anti-Leishmania immunity known. Here, we briefly review the different approaches to vaccination against cutaneous leishmaniasis and argue that vaccines composed of genetically modified (attenuated) parasites, which induce immunity akin to infection-induced resistance, may provide best protection against cutaneous leishmaniasis in humans.