Center for Tropical and Emerging Global Diseases
University of Georgia
Rick Tarleton was recently interviewed for the Drug Discovery News podcast. Listen as he talks about his research into new drug treatments for Chagas disease.
A written transcript is available on DDN’s website.
Rick Tarleton, along with Drew Etheridge‘s lab, was featured in a KFF Health News story about Chagas disease that has been picked up by a number of media outlets.
KFF Health News (Spanish translation)
Host cell invasion by Trypanosoma cruzi is a markedly silent process, with limited host transcriptional changes indicative of innate immune recognition, except for a modest type I IFN (IFN-I) response. In this study, we show that T. cruzi-induced IFN-β production was nearly abolished in primary murine cGAS-/- or stimulator of IFN genes (STING)-deficient (STINGGt) macrophages and fibroblasts. T. cruzi infection did not impact the ability of IFN-regulatory factor reporter macrophages to respond to classical cGAS-STING agonists, indicating that the limited IFN-β induction is not due to active parasite suppression. cGAS-/-, STINGGt, and IFN-α/β receptor-/- (IFNAR-/-) macrophages infected with T. cruzi yielded significantly higher numbers of amastigotes compared with wild-type macrophages; however, the impact of the STING pathway during infection in vivo is more complex. Despite an initial increase in parasite growth, STINGGt and IFNAR-/- mice ultimately had lower parasite burden in footpads as compared with wild-type mice, demonstrating a role for IFN-I expression in potentiating parasite growth at the infection site. STING pathway activation had little impact on parasite levels in the skeletal muscle; however, in the heart, cGAS-/- and STINGGt mice, but not IFNAR-/- mice, accumulated higher acute parasite loads, suggesting a protective role of STING sensing of T. cruzi in this organ that was independent of IFN-I. Together, these results demonstrate that host cGAS-STING senses T. cruzi infection, enhancing parasite growth at the site of entry, and contributes to acute-phase parasite restriction in the heart, a major site of tissue damage in chronic T. cruzi infection.
Natasha Perumal, Brooke White, Fernando Sanchez-Valdez, Rick L Tarleton. J Immunol. 2023 Aug 21;ji2300373. doi: 10.4049/jimmunol.2300373.
Trypanosoma cruzi, the causative agent of Chagas disease, is a zoonotic, vector-borne, protozoan hemoflagellate with a wide host range. An 11-yr-old, captive-bred male De Brazza’s monkey (Cercopithecus neglecus) presented with weight loss despite normal appetite. Examination revealed hypoglycemia, nonregenerative anemia, and many trypanosomes on a blood smear. A whole blood sample was PCR-positive for T. cruzi discrete typing unit TcIV and the monkey seroconverted using two different methods. The monkey was treated with the standard human dose of benznidazole twice daily for 60 d; however, blood obtained over the next 1.5 yr posttreatment remained PCR-positive for T. cruzi. A second course of benznidazole at a higher dose but lower frequency for 26 wk was required for the monkey to convert to sustained PCR-negative status. The monkey recovered with no apparent lasting effects.
Stephanie McCain, Richard R Sim, Bridget Weidner, Anne E Rivas, Brooke White, Lisa D Auckland, Rick L Tarleton, Sarah Hamer. J Zoo Wildl Med. 2023 Jul;54(2):412-416. doi: 10.1638/2022-0095
The Chagas field has gone >50 years without tangible progress toward new therapies. My colleagues and I have recently reported on a benzoxaborole compound that achieves consistent parasitological cure in experimentally infected mice and in naturally infected non-human primates (NHPs). While these results do not assure success in human clinical trials, they significantly de-risk this process and form a strong justification for such trials. Highly effective drug discovery depends on a solid understanding of host and parasite biology and excellent knowledge in designing and validating chemical entities. This opinion piece seeks to provide perspectives on the process that led to the discovery of AN15368, with the hope that this will facilitate the discovery of additional clinical candidates for Chagas disease.
Rick L. Tarleton. Trends Parasitol. 2023 Jun;39(6):423-431. doi: 10.1016/j.pt.2023.03.015.
Trypanosoma cruzi naturally infects a broad range of mammalian species and frequently results in the pathology that has been most extensively characterized in human Chagas disease. Currently employed treatment regimens fail to achieve parasitological cure of T. cruzi infection in the majority of cases. In this study, we have extended our previous investigations of more effective, higher dose, intermittent administration protocols using the FDA-approved drug benznidazole (BNZ), in experimentally infected mice and in naturally infected dogs and nonhuman primates (NHP). Collectively, these studies demonstrate that twice-weekly administration of BNZ for more than 4 months at doses that are ~2.5-fold that of previously used daily dosing protocols, provided the best chance to obtain parasitological cure. Dosing less frequently or for shorter time periods was less dependable in all species. Prior treatment using an ineffective dosing regimen in NHPs did not prevent the attainment of parasitological cure with an intensified BNZ dosing protocol. Furthermore, parasites isolated after a failed BNZ treatment showed nearly identical susceptibility to BNZ as those obtained prior to treatment, confirming the low risk of induction of drug resistance with BNZ and the ability to adjust the treatment protocol when an initial regimen fails. These results provide guidance for the use of BNZ as an effective treatment for T. cruzi infection and encourage its wider use, minimally in high value dogs and at-risk NHP, but also potentially in humans, until better options are available.
Juan M Bustamante, Brooke E White, Gregory K Wilkerson, Carolyn L Hodo, Lisa D Auckland, Wei Wang, Stephanie McCain, Sarah A Hamer, Ashley B Saunders, Rick L Tarleton. Antimicrob Agents Chemother. 2023 Apr 11;e0013223. doi: 10.1128/aac.00132-23.
Although parasite entry through breaks in the skin or mucosa is one of the main routes of natural transmission of Trypanosoma cruzi, little is known about the host cell types initially invaded nor the ability of those host cells to initiate immune responses at the site of infection. To gain insights into these early events, we studied the fate of fluorescently tagged T. cruzi delivered subcutaneously in mouse footpads or ears. We demonstrate that the majority of parasites introduced into the skin initially proliferate there until 8 to 10 days postinfection, when the parasite load decreases. This decline in parasite numbers is dependent on the presence of an intact T cell compartment and on the ability of hosts to produce gamma interferon (IFN-γ). Many of the parasite-containing cells at the initial infection site display a macrophage/monocyte phenotype but with low expression of activation markers, suggesting that these cells provide an early niche for T. cruzi proliferation, rather than being active in parasite control. It is only after the first round of T. cruzi replication and release from host cells that signs of immune activation and control of parasites become apparent. The delay in the activation and failure to rapidly control parasite replication are observed even when T. cruzi-primed T cells are present, such as in chronically infected mice. This failure of a primed immune system to recognize and react prior to extensive parasite expansion at the infection site likely poses a significant challenge for the development of vaccines aiming to prevent T. cruzi infection. IMPORTANCE Trypanosoma cruzi, the parasite causing Chagas disease, usually infects through the mucosa or breaks in the skin, but little is known about the parasite’s fate at the site of entry or the early events involving immune control there. Here, we track the local proliferation and subsequent dissemination of fluorescently tagged T. cruzi and the initial immune response at the point of entry. We show that T. cruzi preferentially infects innate immune cells in the skin and that the stimulation of an adaptive T cell response does not occur until after the release of parasites from this first round of infected host cells. This first immunologically “silent” proliferation occurs even in the presence of a strong immune T cell memory generated by previous infection. This capacity of T. cruzi to establish infections while avoiding initial immune recognition has important implications for the potential to develop vaccines to prevent T. cruzi infection.
Angel M Padilla, Charles Rosenberg, Peter Cook, Fernando Sanchez-Valdez, Caroline McElhannon, Rick L Tarleton. mSphere. 2023 Jan 25;e0060122. doi: 10.1128/msphere.00601-22.
Infection with the protozoan parasite Trypanosoma cruzi elicits substantial CD8+ T cell responses that disproportionately target epitopes encoded in the large trans-sialidase (TS) gene family. Within the C57BL/6 infection model, a significant proportion (30-40%) of the T. cruzi-specific CD8+ T cell response targets two immunodominant TS epitopes, TSKb18 and TSKb20. However, both TS-specific CD8+ T cell responses are dispensable for immune control, and TS-based vaccines have no demonstrable impact on parasite persistence, a determinant of disease. Besides TS, the specificity and protective capacity of CD8+ T cells that mediate immune control of T. cruzi infection are unknown. With the goal of identifying alternative CD8+ T cell targets, we designed and screened a representative set of genome-wide, in silico-predicted epitopes. Our screen identified a previously uncharacterized, to our knowledge, T cell epitope MUCKb25, found within mucin family proteins, the third most expanded large gene family in T. cruzi. The MUCKb25-specific response was characterized by delayed kinetics, relative to TS-specific responses, and extensive cross-reactivity with a large number of endogenous epitope variants. Similar to TS-specific responses, the MUCKb25 response was dispensable for control of the infection, and vaccination to generate MUCK-specific CD8+ T cells failed to confer protection. The lack of protection by MUCK vaccination was partly attributed to the fact that MUCKb25-specific T cells exhibit limited recognition of T. cruzi-infected host cells. Overall, these results indicate that the CD8+ T cell compartment in many T. cruzi-infected mice is occupied by cells with minimal apparent effector potential.
Molly E Bunkofske, Natasha Perumal, Brooke White, Eva-Maria Strauch, Rick Tarleton. J Immunol. 2023 Jan 4;ji2200723. doi: 10.4049/jimmunol.2200723.
Background: A drawback in the treatment of chronic Chagas disease (American trypanosomiasis) is the long time required to achieve complete loss of serological reactivity, the standard for determining treatment efficacy.
Methods: Antibody-secreting and memory B cells specific for Trypanosoma cruzi and their degree of differentiation were evaluated in adult and pediatric subjects with chronic Chagas disease prior to and after etiological treatment.
Results: Trypanosoma cruzi-specific antibody-secreting cells disappeared from the circulation in benznidazole or nifurtimox-treated subjects with declining parasite-specific antibody levels posttreatment, whereas B cells in most subjects with unaltered antibody levels were low prior to treatment and did not change after treatment. The timing of the decay in parasite-specific antibody-secreting B cells was similar to that in parasite-specific antibodies as measured by a Luminex-based assay, but preceded the decay in antibody levels detected by conventional serology. The phenotype of total B cells returned to a non-infection profile after successful treatment.
Conclusions: T. cruzi-specific antibodies in the circulation of chronically T. cruzi-infected subjects likely derive from both antigen-driven plasmablasts, that disappear following successful treatment, and long-lived plasma cells that persist and account for the low frequency and long course to complete seronegative conversion in successfully treated subjects.
G Cesar, M A Natale, M C Albareda, M G Alvarez, B Lococo, Ana María De Rissio, Marisa Fernandez, M Castro Eiro, G Bertocchi, B E White, F Zabaleta, R Viotti, R L Tarleton, S A Laucella.J Infect Dis. 2022 Dec 26;jiac495. doi: 10.1093/infdis/jiac495. Online ahead of print.
Human clinical trials are expensive, and when they fail, they create the impression that a problem is intractable, thus depressing interest in future attempts. For neglected tropical diseases, where there are likely limited numbers of “shots on goal”, such failures need to be assiduously avoided. Chagas disease drug discovery efforts have experienced more than its share of human clinical trial failures. Here Here are some guidelines, many specific for Chagas, but some which might also have application for other neglected tropical diseases. Chagas disease has major challenges (e.g., the lack of a definitive test of cure) but also has outstanding advantages, among these the unmatched multi-species natural infection systems that can be exploited to de-risk compounds before human trials. Fully utilizing these advantages while frankly acknowledging and addressing the challenges should bring better options to patients, sooner.
Rick Tarleton. Clin Infect Dis. 2022 Nov 14;ciac884. doi: 10.1093/cid/ciac884. Online ahead of print.
Trypanosoma cruzi naturally infects a wide variety of wild and domesticated mammals, in addition to humans. Depending on the infection dose and other factors, the acute infection can be life-threatening, and in all cases, the risk of chagasic heart disease is high in persistently infected hosts. Domestic, working, and semi-feral dogs in the Americas are at significant risk of T. cruzi infection and in certain settings in the southern United States, the risk of new infections can exceed 30% per year, even with the use of vector control protocols. In this study, we explored whether intermittent low-dose treatment with the trypanocidal compound benznidazole (BNZ) during the transmission season, could alter the number of new infections in dogs in an area of known, intense transmission pressure. Preliminary studies in mice suggested that twice-weekly administration of BNZ could prevent or truncate infections when parasites were delivered at the mid-point between BNZ doses. Pre-transmission season screening of 126 dogs identified 53 dogs (42.1%) as T. cruzi infection positive, based upon blood PCR and Luminex-based serology. Serial monitoring of the 67 uninfected dogs during the high transmission season (May to October) revealed 15 (22.4%) new infections, 6 in the untreated control group and 9 in the group receiving BNZ prophylaxis, indicating no impact of this prophylaxis regimen on the incidence of new infections. Although these studies suggest that rigorously timed and more potent dosing regimen may be needed to achieve an immediate benefit of prophylaxis, additional studies would be needed to determine if drug prophylaxis reduced disease severity despite this failure to prevent new infections.
Juan M Bustamante, Angel M Padilla, Brooke White, Lisa D Auckland, Rachel E Busselman, Stephanie Collins, Elizabeth L Malcolm, Briana F Wilson, Ashley B Saunders, Sarah A Hamer, Rick L Tarleton. PLoS Negl Trop Dis. 2022 Oct 31;16(10):e0010688. doi: 10.1371/journal.pntd.0010688.
