Phospholipid related antigens and protective mechanisms : implications for cardiovascular diseases, human autoimmunity and inflammation

The pathogenesis of chronic inflammation, a common denominator in cardiovascular, metabolic and systemic autoimmune diseases, is influenced by a number of genetic, metabolic and immunological risk factors. Oxidation of cellular membranes and lipoproteins by reactive oxygen species generates a number of complex and distinct oxidized phospholipids (OxPL) thought to be involved in this pathogenesis. Since the discovery of OxLDL in atherosclerotic lesions, research on these oxidized phospholipids and protective factors/mechanisms has expanded in recent years. As a result, oxidized phospholipids and related antigens have been characterized extensively in connection not only with cardiovascular pathogenesis, but also with related diseases that also are associated with negative cardiovascular outcomes. As a consequence, these oxidized phospholipids are now known to play a key role not only in initiating inflammation, but also in mediating deleterious consequences.

In our first study, we examined the potential pro-inflammatory features of oxidized cardiolipins, phospholipids present in LDL, where they might be exposed at the surface following oxidation. Initially, we determined whether oxidized cardiolipin (OxCL) exerts pro-inflammatory effects on macrophages and neutrophils and, if so, whether Annexin-A5 can inhibit these effects. As expected, OxCL induced leukotriene production by these immune cells in a calcium-dependent manner through activation of the 5-LOX gene. Moreover, Annexin A5 bound to oxidized, but not native cardiolipin, thereby abrogating both the elevated leukotriene production and even intracellular mobilization of calcium. Annexin A5 also inhibited the expression of adhesion molecules on endothelial cells and might thus play an important role in preventing the binding of lymphocytes and monocytes to these cells and resulting inflammation around atherosclerotic plaques.

In our second investigation, we characterized the clinical significance of antibodies towards malondialdehyde, an important epitope exposed at the surface of both OxLDL and apoptotic cells, in 60-year-old patients with cardiovascular disease. An assay developed to measure circulating levels of anti-MDA IgMs revealed that these patients had lower levels than ageand gender-matched controls, especially the men (130.1 [107.8–155] versus 143 [120–165], p=0.001), RU/mL. The odds ratio and 95% CI below the 10th [2.0; 1.19-3.36] and above the 66th percentile [0.68; 0.48-0.98] indicated that these antibodies can predict risk in such patients. Amino acid sequencing showed less variation in these antibodies than in nonspecific IgMs.

To extend these findings to systemic autoimmune diseases, we next assessed the prevalence of anti-MDA and anti-PC IgM in patients with RA, SLE, SjS, SSc, MCTD, and UCTD. The levels of anti-PC, but not ant-MDA IgM were strikingly lower in the case of MCTD. Moreover, levels of both IgMs were low among those with SLE or Sjogren’s syndrome and high among patients with rheumatoid arthritis and primary phospholipid syndrome, with no difference in the case of UCTD. Furthermore, when anti-PC IgM was added exogenously, the number of immunosuppressive T cells (Tregs) increased, with no such effect with anti-MDA IgM. Finally, the amino acid sequences of these antibodies showed both certain similarities and differences. In our last study, we assessed the prevalence of anti–PC IgG1 and IgG2 in patients with SLE. Low levels of IgG1 proved to be indicators of risk, whereas higher concentrations were protective. In addition, exposure of macrophages to monoclonal antibodies against phosphorylcholine (developed in-house) improved the efficiency of phagocytosis by these cells in a manner dependent on complement, which might explain why higher levels of these antibodies in patients with SLE appear to be protective.

Overall, the clinical and experimental evidence we provide here confirms the relevance of anti-PC and anti-MDA IgMs in connection with cardiovascular and autoimmune diseases. Both these natural protective antibodies and Annexin A5 could serve as prognostic markers and potentially be of value in treating inflammatory and autoimmune conditions.

List of scientific papers

I. Wan M, Hua X, Su J, Thiagarajan D, Frostegård AG, Haeggström JZ, Frostegård J. Oxidized but not native Cardiolipin has pro-inflammatory effects which are inhibited by Annexin A5. Atherosclerosis. 2014 Aug;235(2):592-8.
https://doi.org/10.1016/j.atherosclerosis.2014.05.913

II. Thiagarajan D, Frostegård AG, Singh S, Rahman M, Liu A, Vikström M, Leander K, Gigante B, Hellenius ML, Zhang B, Zubarev RA, de Faire U, Lundström SL, Frostegård J. Human IgM Antibodies to Malondialdehye conjugated with Albumin are Negatively Associated with cardiovascular Diseases among 60-Year-Olds. Journal of American Heart Association. 2016, Dec 20;5(12).
https://doi.org/10.1161/JAHA.116.004415

III. Thiagarajan D, Oparina N, Lundström S, Zubarev R, Sun J, the PRECISESADS Clinical Consortium, Marta Alarcon-Riquelme and Frostegård J. IgM antibodies against malondialdehyde and phosphorylcholine in different rheumatic diseases. [Manuscript]

IV. Thiagarajan D, Frostegård A, Steen J, Rahman M, Vikström M, Zubarev R, Lundström S and Frostegård J. IgG1 antibodies against phosphorylcholine are associated with protection in SLE and atherosclerosis: potential underlying mechanisms. [Manuscript]