Plasma Peptidylarginine Deiminase IV Promotes VWF-Platelet String Formation and Accelerates Thrombosis After Vessel Injury.

RATIONALE: PAD4 (peptidylarginine deiminase type IV), an enzyme essential for neutrophil extracellular trap formation (NETosis), is released together with neutrophil extracellular traps into the extracellular milieu. It citrullinates histones and holds the potential to citrullinate other protein targets. While NETosis is implicated in thrombosis, the impact of the released PAD4 is unknown. OBJECTIVE: This study tests the hypothesis that extracellular PAD4, released during inflammatory responses, citrullinates plasma proteins, thus affecting thrombus formation. METHODS AND RESULTS: Here, we show that injection of r-huPAD4 in vivo induces the formation of VWF (von Willebrand factor)-platelet strings in mesenteric venules and that this is dependent on PAD4 enzymatic activity. VWF-platelet strings are naturally cleaved by ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type-1 motif-13). We detected a reduction of endogenous ADAMTS13 activity in the plasma of wild-type mice injected with r-huPAD4. Using mass spectrometry and in vitro studies, we found that r-huPAD4 citrullinates ADAMTS13 on specific arginine residues and that this modification dramatically inhibits ADAMTS13 enzymatic activity. Elevated citrullination of ADAMTS13 was observed in plasma samples of patients with sepsis or noninfected patients who were elderly (eg, age >65 years) and had underlying comorbidities (eg, diabetes mellitus and hypertension) as compared with healthy donors. This shows that ADAMTS13 is citrullinated in vivo. VWF-platelet strings that form on venules of Adamts13-/- mice were immediately cleared after injection of r-huADAMTS13, while they persisted in vessels of mice injected with citrullinated r-huADAMTS13. Next, we assessed the effect of extracellular PAD4 on platelet-plug formation after ferric chloride-induced injury of mesenteric venules. Administration of r-huPAD4 decreased time to vessel occlusion and significantly reduced thrombus embolization. CONCLUSIONS: Our data indicate that PAD4 in circulation reduces VWF-platelet string clearance and accelerates the formation of a stable platelet plug after vessel injury. We propose that this effect is, at least in part, due to ADAMTS13 inhibition.

A novel human autoantigen, endothelial cell growth factor, is a target of T and B cell responses in patients with Lyme disease.

OBJECTIVE: Autoantigen presentation by HLA-DR molecules is thought to be a central component of many autoimmune diseases, but identifying disease-relevant autoantigens has been a difficult challenge. In this study we aimed to identify autoantigens in patients with antibiotic-refractory Lyme arthritis, in which infection-induced autoimmunity is thought to play an important role. METHODS: Using tandem mass spectrometry, naturally presented HLA-DR self peptides from a patient's synovium were identified, synthesized, and reacted with his peripheral blood mononuclear cells (PBMCs). Immunoreactive peptides and their source proteins were then tested for T and B cell responses using large numbers of patient cells or sera. RESULTS: Of 120 HLA-DR-presented self peptides identified from one patient, one peptide derived from endothelial cell growth factor (ECGF) caused his PBMCs to proliferate. T and B cell responses to ECGF occurred systemically in ∼10-30% of patients with early or late manifestations of Lyme disease, primarily in those with refractory arthritis-associated HLA-DR alleles, such as DRB1*0101 and 0401. Compared with patients with antibiotic-responsive arthritis, those with antibiotic-refractory arthritis had significantly higher concentrations of ECGF in synovial fluid (P<0.0001) and more often had ECGF antibody reactivity. Among non-antibiotic-treated historical patients who developed arthritis, 26% had ECGF reactivity, which often developed before the onset of arthritis and was associated with significantly longer courses of arthritis. CONCLUSION: T and B cell responses to ECGF occur in a subset of patients with Lyme disease, particularly in those with antibiotic-refractory arthritis, providing the first direct evidence of autoimmune T and B cell responses in this illness.

Peptides presented by HLA-DR molecules in synovia of patients with rheumatoid arthritis or antibiotic-refractory Lyme arthritis.

Disease-associated HLA-DR molecules, which may present autoantigens, constitute the greatest genetic risk factor for rheumatoid arthritis (RA) and antibiotic-refractory Lyme arthritis (LA). The peptides presented by HLA-DR molecules in synovia have not previously been defined. Using tandem mass spectrometry, rigorous database searches, and manual spectral interpretation, we identified 1,427 HLA-DR-presented peptides (220-464 per patient) from the synovia of four patients, two diagnosed with RA and two diagnosed with LA. The peptides were derived from 166 source proteins, including a wide range of intracellular and plasma proteins. A few epitopes were found only in RA or LA patients. However, two patients with different diseases who had the same HLA allele had the largest number of epitopes in common. In one RA patient, peptides were identified as originating from source proteins that have been reported to undergo citrullination under other circumstances, yet neither this post-translational modification nor anti-cyclic citrullinated peptide antibodies were detected. Instead, peptides with the post-translational modification of S-cysteinylation were identified. We conclude that a wide range of proteins enter the HLA-DR pathway of antigen-presenting cells in the patients' synovial tissue, and their HLA-DR genotype, not the disease type, appears to be the primary determinant of their HLA-DR-peptide repertoire. New insights into the naturally presented HLA-DR epitope repertoire in target tissues may allow the identification of pathogenic T cell epitopes, and this could lead to innovative therapeutic interventions.

Strong IgG antibody responses to Borrelia burgdorferi glycolipids in patients with Lyme arthritis, a late manifestation of the infection.

In this study, the membrane lipids of B. burgdorferi were separated into 16 fractions; the components in each fraction were identified, and the immunogenicity of each fraction was determined by ELISA using sera from Lyme disease patients. Only the 2 glycolipids, acylated cholesteryl galactoside (ACG, BbGL-I) and monogalactosyl diacylglycerol (MgalD, BbGL-II), were immunogenic. Early in the infection, 24 of 84 patients (29%) who were convalescent from erythema migrans and 19 of the 35 patients (54%) with neuroborreliosis had weak IgG responses to purified MgalD, and a smaller percentage of patients had early responses to synthetic ACG. However, almost all of 75 patients with Lyme arthritis, a late disease manifestation, had strong IgG reactivity with both glycolipids. Thus, almost all patients with Lyme arthritis have strong IgG antibody responses to B. burgdorferi glycolipid antigens.

Phosphorylation of the pro-apoptotic protein Bim in lymphocytes is associated with protection from apoptosis.

Bim is a pro-apoptotic member of the Bcl-2 protein family. Bim has three isoforms, EL, L, and S, of which the EL form is the least cytotoxic. We show here that Bim is serine phosphorylated in lymphocytes, predominantly on the EL form. Withdrawal of IL-2 from IL-2-dependent T lymphocytes or culture of thymocytes leads to reduced Bim phosphorylation and apoptosis induction. This decrease in Bim phosphorylation occurs when most cells in culture are still viable, indicating that reduction of Bim phosphorylation may be an early event in apoptosis signaling of lymphocytes.