Richard G. Titus

Sandfly Maxadilan Exacerbates Infection with Leishmania major and Vaccinating Against It Protects Against L. major Infection

Bloodfeeding arthropods transmit many of the world’s most serious infectious diseases. Leishmania are transmitted to their mammalian hosts when an infected sandfly probes in the skin for a bloodmeal and injects the parasite mixed with its saliva. Arthropod saliva contains molecules that affect blood flow and modulate the immune response of the host. Indeed, sandfly saliva markedly enhances the infectivity of L. major for its host. If the salivary molecule(s) responsible for this phenomenon was identified, it might be possible to vaccinate the host against this molecule and thereby protect the host against infection with Leishmania. Such an approach represents a novel means of controlling arthropod-borne disease transmission. Here, we report that a single molecule, maxadilan, in sandfly saliva can exacerbate infection with L. major to the same degree as whole saliva, and that vaccinating against maxadilan protects mice against infection with L. major.

The immunomodulatory factors of arthropod saliva and the potential for these factors to serve as vaccine targets to prevent pathogen transmission

In general, attempts to develop vaccines for pathogens transmitted by arthropods have met with little or no success. It has been widely observed that the saliva of arthropods that transmit disease enhances the infectivity of pathogens the arthropod transmits to the vertebrate host. Indeed, it has been observed that vaccinating against components of the saliva of arthropods or against antigens expressed in the gut of arthropods can protect the host from infection and decrease the viability of the arthropod. These results suggest that multi‐subunit vaccines that target the pathogen itself as well as arthropod salivary gland components and arthropod gut antigens may be the most effective at controlling arthropod‐borne pathogens as these vaccines would target several facets of the lifecycle of the pathogen. This review covers known immunomodulators in arthropod salivary glands, instances when arthropod saliva has been shown to enhance infection and a limited number of examples of antiarthropod vaccines, with emphasis on three arthropods: sandflies, mosquitoes and hard ticks.

The immunomodulatory factors of bloodfeeding arthropod saliva

There are potent immunomodulators in saliva of the bloodfeeding arthropods which transmit many of the worlds most serious diseases that may benefit the arthropod by preventing the vertebrate host from becoming sensitized to the saliva. In addition, saliva can enhance transmission of parasites/pathogens by arthropods. As a result, vaccines that target the arthropod (e.g. salivary immunomodulators) should be considered as one component of multisubunit vaccines against arthropod‐borne parasites/pathogens. Indeed, since vaccines against the pathogens themselves are often not fully protective, vaccines that target several facets of the life cycle of the pathogen may be the most effective at controlling disease transmission. This review covers known immunomodulatory factors in arthropod vector saliva, focusing mainly on sandflies and ixodid ticks.

The role of vector saliva in transmission of arthropod-borne disease.

Blood-sucking arthropod disease vectors all share one important feature: while probing for blood in the vertebrate hosts skin they salivate into the wound they create. Recent studies on the pharmacological properties of vector saliva have revealed an array of activities that are potentially beneficial to both the vector and to the pathogen. These observations may help explain why certain vectors and pathogens have co-evolved. In this article, Richard Titus and Jose Ribeiro discuss the role vector saliva may play in disease transmission, and the prospects for its use in the control of arthropod-borne pathogens.

A simple method for quantifying Leishmania in tissues of infected animals

In experimental animals infected with Leishmania major, the size of cutaneous lesions of the parasite often does not correlate with the number of parasites within the lesion. Indeed, cutaneous lesions can heal, but still contain parasites. Thus, the ability to determine parasite burden in infected animals becomes important, especially when assessing vaccines that are intended to induce sterilizing immunity. Here, Hermenio Lima, Julie Bleyenberg and Richard Titus describe a simple technique for enumerating Leishmania in infected tissue. It is hoped that this technique will allow all researchers working with Leishmania (especially those in countries where leishmaniasis is endemic) to determine parasite burden easily in infected animals.

Identification of an IL-2 Binding Protein in the Saliva of the Lyme Disease Vector Tick, Ixodes scapularis

A potent inhibitor of mitogen-stimulated T cell proliferation exists in the saliva of several species of hard ticks, including the Lyme disease vector tick, Ixodes scapularis. Our characterization of this phenomenon has led to the identification of a possible mechanism for the T cell inhibitory activity of I. scapularis saliva. The T cell inhibitor can overcome stimulation of mouse spleen cells with anti-CD3 mAb; however, a direct and avid interaction with T cells does not appear to be necessary. Tick saliva inhibits a mouse IL-2 capture ELISA, suggesting that a soluble IL-2 binding factor is present in the saliva. This hypothesis was verified by using a direct binding assay in which plate-immobilized tick saliva was shown to bind both mouse and human IL-2. Elimination of the IL-2 binding capacity of saliva in the in vitro assays by trypsin digestion demonstrated that the IL-2 binding factor is a protein. These experiments comprise the first demonstration of the existence of such a secreted IL-2 binding protein from any parasite or pathogen. This arthropod salivary IL-2 binding capacity provides a simple mechanism for the suppression of T cell proliferation as well as for the activity of other immune effector cells that are responsive to IL-2 stimulation. Relevance of the tick T cell inhibitory activity to the human immune system is demonstrated by the ability of tick saliva to inhibit proliferation of human T cells and CTLL-2 cells grown in the presence of human IL-2.

Salivary gland material from the sand fly Lutzomyia longipalpis has an inhibitory effect on macrophage function in vitro

Previous work from our laboratory demonstrated that the infectivity of the protozoan parasite Leishmania major was enhanced in mice if the infecting inoculum contained salivary gland lysates from the sand fly vector Lutzomyia longipalpis. The present study was designed to address the hypothesis that sand fly salivary gland material may function by inhibiting the host immune response. Results indicated that sand fly saliva inhibited the ability of macrophages to present leishmanial antigens to parasite‐specific T cells.

Local Suppression of T Cell Responses by Arginase-Induced L-Arginine Depletion in Nonhealing Leishmaniasis

The balance between T helper (Th) 1 and Th2 cell responses is a major determinant of the outcome of experimental leishmaniasis, but polarized Th1 or Th2 responses are not sufficient to account for healing or nonhealing. Here we show that high arginase activity, a hallmark of nonhealing disease, is primarily expressed locally at the site of pathology. The high arginase activity causes local depletion of L-arginine, which impairs the capacity of T cells in the lesion to proliferate and to produce interferon-γ, while T cells in the local draining lymph nodes respond normally. Healing, induced by chemotherapy, resulted in control of arginase activity and reversal of local immunosuppression. Moreover, competitive inhibition of arginase as well as supplementation with L-arginine restored T cell effector functions and reduced pathology and parasite growth at the site of lesions. These results demonstrate that in nonhealing leishmaniasis, arginase-induced L-arginine depletion results in impaired T cell responses. Our results identify a novel mechanism in leishmaniasis that contributes to the failure to heal persistent lesions and suggest new approaches to therapy.

The involvement of TNF, IL-1 and IL-6 in the immune response to protozoan parasites.

One early reaction of the host to infection with protozoan parasites is the secretion of an array of potent cytokines including tumor necrosis factor (TNF), interleukin (IL-1) and IL-6. The combined action of these cytokines causes fever, leukocytosis and the production of acute phase proteins such as C-reactive protein (CRP). These early responses contribute significantly to the outcome of infection by influencing the course of infection directly and by regulating the specific immune response to the parasite.

Histologic characterization of experimental cutaneous leishmaniasis in mice infected with Leishmania braziliensis in the presence or absence of sand fly vector salivary gland lysate.

Leishmania braziliensis is the causative agent of human cutaneous leishmaniasis in parts of the New World. In the murine model of infection, L. braziliensis does not produce severe or lasting cutaneous lesions in either BALB/c or C3H mice. However, when the parasites are injected into BALB/c mice with salivary gland lysate of the sand fly vector for the parasite, infection is significantly enhanced, as measured by lesion size, parasite burden, and the outcome of infection. Histologic examination of these cutaneous lesions showed that initially, nodular and diffuse dermal infiltrates of neutrophils, eosinophils, and histiocytes occurred in all mice. Over time, the saliva-free lesions progressed to small organized granulomas of epithelioid macrophages that contained few parasites, with eventual resolution of inflammation and mild dermal fibrosis. The saliva-associated lesions progressed to extensive, poorly organized accumulations of heavily parasitized epithelioid macrophages, with persistent neutrophils and eosinophils, and minimal fibroplasia. These results indicate that sand fly salivary gland lysate markedly modifies the inflammatory response to infection with L. braziliensis.