Though great strides have been made in improving the survival of patients with lupus, the ultimate etiology of the disease remains a mystery. Complex disease course, differing response to medications and patterns of flare and inactivity conspire to make lupus — as described by the National Institute of Arthritis and Musculoskeletal and Skin Diseases — "the most heterogenous of the autoimmune diseases."
Multiple lines of molecular research in mouse models have chipped away at lupus' mysteries. One angle is the link between the disease and difficulties with clearing apoptotic debris in the body. This debris has long been observed in lupus models, but new research hints that it might play a causal role in the development of flares.
Barbara Vilen, Ph.D., an associate professor of microbiology and immunology at the University of North Carolina, Chapel Hill, recently received a grant from the Lupus Research Institute to investigate apoptotic debris in human lupus flares.
In this Q&A with Rheumatology Network, Dr. Vilen, discusses her research.
RN: Tell me about the animal research that led to this new line of study.
Dr. Vilen: We found that the murine lysosome fails to fully mature and as a consequence, it can't acidify. Thus, the cargo brought into the cell by phagocytosis can't be degraded and as a consequence, it recycles back to the cell surface. It takes 48-72 hours for the cell to recycle the internalized cargo and during this time, the phagolysosomal membrane becomes permeabalized. So in addition to recycling the nuclear antigens back to the cell surface, some of the contents of the lysosome (including DNA and antibody) leak into the cytosol. There are a number of cytosolic sensors that normally provide host protection against invading pathogens. These cytosolic sensors recognize pathogens associated molecular patterns (PAMPs) induce interferon-alpha to clear the infection. In lupus, the diminished degradation prolongs access of nucleic acid to toll-like receptors and the leaking of cargo into the cytosol allows DNA from the apoptotic debris, and antibodies, to engage cytosolic sensors (AIM2 and TRIM21). This leads to autoantibody and interferon-alpha production.
RN: Is this something that you would say is the root cause of lupus, or part of the cause of lupus?
Dr. Vilen: It’s likely just part of the underlying pathology and perhaps, it may just be defective in a subset of patients. We don’t know yet. Our murine data indicates that diminished lysosomal maturation is upstream of several of the lupus-relevant pathologies including B-cell expansion, autoantibody, BAFF interferon-alpha, pyroptosis, and lupus nephritis.
Thus, understanding the molecular basis for the diminished lysosomal maturation could prove beneficial in identifying targetable events that might correct multiple pathologies. I don’t believe there will be one cause of lupus. It is clearly multigenic, environmentally influenced and thus, likely develops from multiple etiologies.
RN: How have you gone about moving this research into humans?
Dr. Vilen: We can get lots of information from murine samples about lupus, but after 20 years, we haven't gotten a cure or very many good treatments. We moved to clinical research very quickly after our original observations with hopes of assessing whether we could see the accumulation of apoptotic debris in any disease state, and whether the accumulation of IgG-ICs waxes and wanes in different states of disease.
The study that we recently published included a pilot study with 8 or 9 active patients (SLEDAI >6). We stained peripheral blood mononuclear cells for the nuclear antigens and IgG and found that 67% - 75 percent of the people in flare showed the accumulation of nuclear antigen and IgG. We found that approximately 35% of the non-flare patients show apoptotic debris on the surface. Whether the accumulated IgG-ICs in patients with inactive disease are residual from a previous flare remains unclear since we have not established the kinetics of accumulation and how the ICs clear the cell surface once active disease recedes.
We are currently enrolling more patients with active and inactive disease. It's very hard to follow these patients. They're chronically ill, so they don't typically like to come to their doctor's appointments much less come every week or twice a week for blood draws.
RN: What is your next step to tease this out further in human disease?
Dr. Vilen: We are following patients longitudinally to address whether the levels of IgG-ICs increase and decrease during active and inactive disease states in the same individual.
We are also continuing the murine studies. This is truly a mouse to human to mouse project. We want to identify the molecular events that induce the lysosomal maturation defect. That will allow us to start looking at targets that might correct the defect. We can then go back to human and assess whether the murine defects are evident in flaring patients. This will allow us to assess therapeutic possibilities.
RN: How does this research help understand the basic science of lupus?
Dr. Vilen: There's been a long association of apoptotic debris in lupus. One of the first lupus publications in the late '40s talked about this LE cell identified in bone marrow, later identified as a macrophage that had engulfed apoptotic debris. This set in motion research looking at the association of apoptotic debris and immune complexes in lupus.
Our research advances what has been known about apoptotic debris, IgG-ICs and lupus in that it reveals an inability to degrade apoptotic debris and IgG-ICs formed from autoantibody and apoptotic debris. This, at least in part, explains the heightened levels of apoptotic debris and IgG-ICs in human and murine disease. It also connects known lupus pathologies, such as B cell expansion, autoantibody, BAFF, interferon-alpha, the cell death, with diminished lysosomal maturation and provides a molecular means who failure to degrade IgG-ICs in macrophages contributes to disease pathology.
Admittedly, it is a very complicated story and I think many dismiss it, however, they fail to realize that previous studies may have missed the bullseye in understanding how IgG-ICs and apoptotic debris contribute to SLE.
Kang S, Rogers JL, Monteith AJ, et al. Apoptotic Debris Accumulates on Hematopoietic Cells and Promotes Disease in Murine and Human Systemic Lupus Erythematosus. The Journal of Immunology. 2016;196(10):4030-4039. doi:10.4049/jimmunol.1500418.
Monteith AJ, Kang S, Scott E, et al. Defects in lysosomal maturation facilitate the activation of innate sensors in systemic lupus erythematosus. Proceedings of the National Academy of Sciences Proc Natl Acad Sci USA. 2016;113(15). doi:10.1073/pnas.1513943113.
Pathak S, Mohan C. Cellular and molecular pathogenesis of systemic lupus erythematosus: lessons from animal models. Arthritis Research & Therapy. 2011;13(5):241. doi:10.1186/ar3465.