SCARF1 in the Pathogenesis of Systemic Lupus Erythematosus
April Jorge1, Taotao Lao1, Terry K. Means1,2, Zaida G. Ramirez-Ortiz1,3
1 Massachusetts General Hospital. Department of Rheumatology, Allergy and Immunology. Charlestown, MA 02129
2 Sanofi Immunology and Inflammation Research Therapeutic Area. Cambridge, MA 02139
3 University of Massachusetts Medical School. Department of Medicine. Worcester MA 01605


Defects at the cellular level in the detection and clearance of apoptotic cells (ACs) contribute to the pathogenesis of Systemic Lupus Erythematosus (SLE). We previously identified Scavenger Receptor Class F 1 (SCARF1) expressed on dendritic cells (DCs) where it functions as a receptor for C1q and mediates capture and engulfment of apoptotic cells. Deficiency in SCARF1 results in impaired removal of ACs (Figure 1) SCARF1 deficient mice develop a lupus-like autoimmune disease with symptoms similar to human SLE, such as production of anti-nuclear and anti-chromatin antibodies, nephritis and dermatitis. However, the role of human SCARF1 in the removal of ACs is unknown. We hypothesize a dysregulation of SCARF1 in SLE patients results in the accumulation of ACs and contributes to SLE disease activity. In order to identify which cells express SCARF1, we used flow cytometry from healthy donors PBMCs. We found that SCARF1 was highly expressed on DCs and monocytes. Treatment with ACs results in a significant decrease in the cell surface expression of SCARF1. Furthermore, using Nanostring and qPCR, we observed an upregulation in autophagy, TLR, and anti-inflammatory genes in cells expressing high levels of SCARF1. Next, we tested whether soluble SCARF1 (sSCARF1) can interact with ACs. Opsonizing ACs for 30 min with recombinant SCARF1 results in an upregulation of SCARF1 expression measured by flow cytometry and qPCR. Lastly, we obtained serum and PBMCs from SLE patient samples. First, we measured the expression of SCARF1 by flow cytometry in the PMBCs. Unexpectedly, there was no significant difference in SCARF1 expression in the SLE patients compared to healthy donors. Next, we measured the concentration of sSCARF1 in the serum by ELISA. We observed a significant increase in sSCARF1 in the SLE patients compared to control patients. We also detected anti-SCARF1 autoantibodies in 26% of SLE patients, which was associated with dsDNA antibody positivity and higher SLE disease activity. Our data demonstrates that human SCARF1 is an ACs receptor in DCs, playing a role in maintaining tolerance and homeostasis.

Alejandro Rodriguez Gama1, Tejbir Kandola1, Shriram Venkatesan1, Jianzheng Wu1,2, Minling Hu1, and Randal Halfmann1,2
1Stowers Institute for Medical Research, Kansas City, MO, USA,
2Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, KS, USA
    
Multiple signaling proteins of the innate immune system exert their cellular activities by assembling into large macromolecular complexes known as signalosomes. We previously discovered that two such proteins – the pyroptosome scaffold ASC, and the antiviral signaling protein MAVS – each assemble through self-templating polymerization that is reminiscent of infectious protein particles known as prions 1. More recently we revealed that prion-like activity broadly arises from structurally encoded kinetic barriers to nucleation – the probabilistic formation of a self-templating multimer de novo 2. For proteins like MAVS and ASC, the nucleation barrier is so high that their soluble inactive states persist despite physiological concentrations that are highly supersaturated with respect to the assembled active state. To identify other innate immune signaling proteins that may function in this manner, we used DAmFRET, a flow cytometric cell-based assay of nucleation barriers 2, to screen 138 candidate prion-like modules from 129 human proteins that function in programmed cell death and innate immune signaling. We discovered 36 of these proteins that are inherently capable of supersaturation and switch-like self-templating activation in living cells. We have further discovered a network of nucleating interactions between them, wherein polymerization of one protein nucleates the polymerization of specific additional proteins. This widespread kinetic control over cell fate indicates that cells are literally waiting to die -- pyroptosis, necroptosis, and alternative cell fates downstream of these proteins are thermodynamically favored, and therefore inevitable with time. I will discuss our investigations into the implications of this phenomenon for aging-associated inflammation and innate immune memory in human monocytes.
1.    Cai, X. et al. Prion-like polymerization underlies signal transduction in antiviral immune defense and inflammasome activation. Cell 156, 1207–1222 (2014).
2.    Khan, T. et al. Quantifying Nucleation In Vivo Reveals the Physical Basis of Prion-like Phase Behavior. Mol. Cell 71, 155-168.e7 (2018).

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada

Recorded during the Keystone Symposia meeting:

Cancer Immunology and Immunotherapy: Taking a Place in Mainstream Oncology

Mar 19 - Mar 23, 2017 | Whistler, British Columbia, Canada