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Comparison of rat and human responses to toll-like receptor 7 activation
Sarah Clarke1, Carl Laxton, Nigel Horscroft
1Drug Safety Research & Development, Pfi zer Global Research and Development, Sandwich, Kent CT13 9NJ, United Kingdom.
Abstract:
Toll-like receptors recognize invading microorganisms and activate innate immune responses. Their discovery has opened up a range of therapeutic possibilities, in particular for infectious diseases. Responses to TLR agonists have been largely studied in mice and little information exists in other species. Given that rats are commonly used for pharmacokinetic and toxicology studies in drug development, we compared TLR7 responses in rat and human. Stimulation of rat and human peripheral blood mononuclear cells with the TLR7 agonist SM360320 showed that in rat cells, the interferon-induced gene, 2', 5' oligoadenylate synthase and tumor necrosis factor alpha were induced at lower concentrations and to a greater degree compared with human cells. Both human and rat cells demonstrated tolerance and could not be restimulated following initial treatment with high concentrations of SM360320. Reducing the concentration of the initial treatment allowed cells to be restimulated following a period of recovery. The initial treatment concentration had to be reduced to a greater extent to enable restimulation of rat cells compared with human cells. Dosing whole rats repeatedly with different concentrations of SM360320 confirmed the in vitro results. Treatment of human cells with high concentrations of interferon alpha did not induce tolerance to subsequent treatment with SM360320 indicating that tolerance occurs in the TLR7 signaling pathway, rather than the interferon signaling pathway. We conclude that rat and human cells respond differently to TLR7 activation and that these differences should be considered when using rat as a model to study TLR7 agonists.
Insights
Rat and human cells show distinct responses to Toll-like Receptor 7 (TLR7) agonists, impacting drug development. Rats exhibit heightened TLR7 activation but develop tolerance more readily than humans, necessitating careful consideration in preclinical studies.
Area of Science:
- Immunology
- Pharmacology
- Toxicology
Background:
- Toll-like receptors (TLRs) are crucial for innate immunity and represent therapeutic targets, particularly for infectious diseases.
- TLR agonist responses are primarily studied in mice, with limited data in other species.
- Rats are frequently used in drug development for pharmacokinetic and toxicology assessments.
Purpose of the Study:
- To compare Toll-like Receptor 7 (TLR7) agonist responses in rat and human cells.
- To investigate potential differences in TLR7-mediated immune activation and tolerance induction between rats and humans.
- To evaluate the utility of rats as a model for studying TLR7 agonists in drug development.
Main Methods:
- Stimulation of rat and human peripheral blood mononuclear cells (PBMCs) with the TLR7 agonist SM360320.
- Measurement of interferon-induced gene, 2', 5' oligoadenylate synthase, and tumor necrosis factor alpha expression.
- Assessment of cellular tolerance and restimulation capacity following varying concentrations of SM360320.
- In vivo validation using repeated dosing of whole rats with SM360320.
Main Results:
- Rat cells exhibited lower concentration thresholds and higher induction levels for key immune mediators compared to human cells upon TLR7 stimulation.
- Both rat and human cells developed tolerance to high concentrations of the TLR7 agonist, with rat cells requiring a greater reduction in initial concentration for restimulation.
- In vivo studies corroborated the in vitro findings regarding differential responses and tolerance induction in rats.
- Interferon alpha treatment did not induce tolerance to TLR7 agonists in human cells, localizing tolerance to the TLR7 pathway.
Conclusions:
- Significant species-specific differences exist in TLR7 signaling pathways between rats and humans.
- These disparities in immune response and tolerance mechanisms must be accounted for when extrapolating rat-based preclinical data to human drug development.
- Rats may not fully recapitulate human TLR7 agonist responses, highlighting the need for cautious interpretation in pharmacokinetic and toxicology studies.
