Saumendra N. Sarkar, PhD
1.8 Hillman Cancer Center
5117 Centre Avenue
Pittsburgh, PA 15213
Innate immunity of an organism is the inborn protection against invading pathogens. Because it is inborn, and entrusted with the protection of host from a vast array of previously unknown invaders, the innate immune system generates a generalized alert response upon pathogen detection. This alert is chemically mediated by a class of molecules called Cytokines. A critical task for this host protection system is to distinguish foreign or non-self, from self, and initiate their destruction or containment. The sensors or the receptors of the innate immune system accomplish this by recognizing specific molecular patterns, which are common to pathogens or pathogen associated molecules, but absent in the host. We focus on a particular subset of these sensors/receptors, which are involved in sensing virus infection.
In order to protect the host from viral invasion, the innate immune system has evolved sensors to detect the viral nucleic acids. Several unique features of virally produced DNA or RNA are exploited to distinguish viral nucleic acids from that of the host. One such unique nucleic acid is double-stranded RNA (dsRNA)—a common byproduct or intermediate in viral genome replication. In mammals, receptors like Toll-like Receptor 3 (TLR3), Retinoic acid-Inducible Gene I (RIG-I) and Melanoma Differentiation-Associated gene-5 (MDA5) are the three known sensors of dsRNA. Single-stranded viral RNA is sensed by Toll-like Receptor 7 and 8 (TLR7 and TLR8), while viral DNA is detected by Toll-like Receptor 9 (TLR9), and other less well characterized receptors.
We study two related aspects of innate immunity: (A) the signaling process involved in cytokine production after virus infection and (B) develop modulators for these signaling pathways. To understand the details of the networks involved in transcriptional induction or ‘switching on’ of genes following the detection of viral nucleic acids, we use various immortalized human cell lines engineered to make TLR3 and TLR7, as model systems. These studies have led us to discover new roles of protein modifications (e.g. tyrosine phosphorylations) involved in TLR3 signaling. We have shown how these phosphorylations control subtle modifications of transcription machinery (transcription factors). Continuing this line of investigation we are now trying to understand how other signaling networks interact and modulate the innate immune signaling pathways in human cells.
In a separate but related project we are identifying new modifiers of innate immune signaling pathways using high throughput screening. Activation of innate immune receptors by their natural microbial agonists leads to up-regulation of a series of pro-inflammatory and cytokine genes to neutralize the infection and shape the subsequent acquired immune response. On the other hand, excess stimulation of the receptor system from invading pathogens or from internal tissue damage products, cause several major inflammatory diseases such as atherosclerosis, asthma, bacterial sepsis and rheumatoid arthritis. Besides chronic inflammatory diseases, innate immune receptors have also been linked to cancer. The most established one is the gastrointestinal malignancy. Epidemiological and genetic evidences have also established links between TLR response and ovarian, prostate, breast and several other cancers. Thus, modulation of innate immune receptor signaling pathways offers an attractive method to fight such diseases. Proofs of this principle have been already established in several cases. Among other TLRs, TLR3 has been shown to mediate inflammation and pathogenesis of viral infection. TLR3 deficient mice are more resistant to lethal infection by West Nile virus than wild type mice. Similarly, TLR3 increases disease morbidity and mortality from Vaccinia and Phlebovirus infection. Therefore, in specific viral infection models, TLR3 may contribute not only to host defense but also to pathogenesis.
We have established cell-based screening systems for dsRNA mediated gene induction. Currently we are screening chemical and genetic modifiers of TLR3 and RIG-I signaling pathway. Additionally, we are in the process of establishing cell-based screening system for TLR7 and TLR8 in order to identify modifiers of their signaling pathways.
Dr. Sarkar conducts his research at the University of Pittsburgh Cancer Institute.
Lulu Shao - Post Doctoral Associate