NEW HOT PAPERS

Why do you think your paper is highly cited? Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper by Christensen et al. is highly cited because it is the first to comprehensively evaluate the roles of Toll-like receptors (TLRs) in the regulation of systemic autoimmunity in a well-characterized murine model. It has long been known that DNA- and RNA-containing molecules are dominant self-antigens in lupus. This is the basis of the classic anti-nuclear antibody (ANA) reaction, a sine qua non of lupus.

However, the reason for the dominant targeting of these autoantigens was not known. In vitro work had suggested that TLRs that recognize nucleic acids, particularly TLR9 that recognizes DNA and TLR7 that recognizes RNA, might be important. Conventionally these TLRs were thought to recognize pathogen-associated forms of these nucleic acids, but we postulated that in autoimmunity they might recognize “self” forms of these molecules.

Would you summarize the significance of your paper in layman’s terms?

We crossed “knockout” alleles of TLR7 and TLR9 onto the lupus-prone genetic background strain MRL.Faslpr. These mice normally make both anti-DNA and anti-RNA autoantibodies. When we made the mutant alleles homozygous, we found that anti-DNA type autoantibodies were absent in the TLR9-deficient mice while anti-RNA type autoantibodies were missing in the TLR7-deficient mice. This directly established the roles of these two TLRs in the generation of humoral autoimmunity.

It might have been predicted that the absence of either TLR would also ameliorate disease, given the entrenched idea that lupus is an autoantibody-mediated disease, and indeed the lack of TLR7 led to reduced pathology and immune activation. However, it turned out that the absence of TLR9 actually exacerbated disease. This unexpected finding indicated that TLR9 played a regulatory role, though the mechanism remains unclear.

Given that TLR7 and TLR9 are expressed in similar tissues and are thought to signal via the same pathway, this divergence is even more intriguing. The findings also had important clinical and drug-development implications, indicating that targeting TLR9 alone for inhibition would not be an ideal strategy. Indeed, a number of companies are developing TLR inhibitors to treat autoimmune diseases including lupus, some of which are already in the clinic.

In general terms, this research helped us to understand why anti-nuclear antibodies are made in lupus. It demonstrated that a part of the immune system that was thought to detect bacteria and virus could be hijacked during autoimmunity to signal an inappropriate immune reaction to one’s own DNA and RNA. At the same time, it guided efforts to develop drugs that would inhibit these same signals and suggested that if the correct receptor were inhibited by such drugs, that lupus could be treated effectively.

How did you become involved in this research, and were there any particular problems encountered along the way?

Our lab has had a longstanding interest in understanding how autoreactive B cells are activated in systemic autoimmune diseases like lupus and also how autoreactive B cells promote disease. Our interest in TLRs was sparked by work of our collaborator Dr. Ann Marshak-Rothstein at Boston University, who made the conceptual breakthrough that TLRs could be important. This work used in vitro assays of autoreactive B cells, focusing on autoreactive B cells that our lab had studied intensively. Since our lab is very adept at in vivo models and disease assessment, we decided to take the next step of investigating “knockouts” of TLRs in murine models of lupus.

Where do you see your research leading in the future?

In the future, we will want to determine if TLR7 and TLR9 are the only receptors involved in lupus. In addition, there could be other so-called “innate immune receptors” that could be involved in this or other autoimmune diseases. A number of groups are now working on these issues. We also want to determine how TLR9 functions as a negative regulator of disease. Key to this will be to understand in which cell types TLR9 and TLR7 function and what the consequences are of their signaling. This will require more complex genetic approaches that we are now developing.

Mark Shlomchik, M.D., Ph.D.
Professor of Laboratory Medicine and Immunobiology
Yale University School of Medicine
New Haven, CT, USA