Antigen Levels of Thrombospondin-1 and ADAMTS13 Are Reduced in Parallel in TTP.

Background Thrombospondin-1 (TSP-1) is considered to control the von Willebrand factor (VWF) multimer size by reducing interchain disulfide bridges, whereas ADAMTS13 is the major VWF cleaving protease. However the exact contribution of each of these to VWF size control is not known. In this study we investigate the relationship between ADAMTS13 antigen levels and TSP-1 levels in diseased plasmas.

Methods Antigen levels of TSP-1 and ADAMTS13 were determined in 21 plasma samples derived from 11 patients with TTP, and compared to normal control, 20 ITP samples and 20 SLE samples. TSP-1 antigen was measured by a commercial kit (Cytimmune, Rockville, MD), which can detect both bound and free TSP-1 in plasma. ADAMTS13 antigen was measured in a newly developed sandwich ELISA using 3 monoclonal antibodies to ADAMTS13. All donors were Chinese.

Results The average TSP-1 antigen level in normal plasma was 18.34±7.24 μ g/ml (mean ± SD, n=30) and the mean ADAMTS13 level in Chinese normals (CN) was 601±145 mU/ml (n=26) compared to the level set at 1 U/ml in pooled (n=20) normal human plasma from Caucasian origin. In TTP plasma, before plasma exchange (pre-PE), 5.79±2.35 μg/ml TSP-1 was detected (n=11, p<0.01 versus CN), which upon PE increased to 9.06±6.52 μ g/ml (n=10, p>0.05 versus pre-PE). The mean antigen level of ADAMTS13 was 98.7±82.8 mU/ml (n=11, p<0.01, versus CN) and 449±232 mU/ml (n=10, p<0.05 versus pre-PE), before and after PE, respectively. Furthermore TSP-1 antigen levels and ADAMTS13 titres in these patients plasma correlated linearly with an R=0.7 (n=21). To determine whether this significant decrease in both TSP-1 and ADAMTS13 antigen levels is restricted to TTP, plasma from ITP and SLE patients was analyzed next, however no significant decrease of TSP-1 was found. In contrast, the average antigen level of ADAMTS13 was raised to 830±229 mU/ml (n=20, p<0.01 versus CN) and 721±192 mU/ml (n=20, p<0.05 versus CN), for ITP and SLE plasmas, respectively.

Conclusion Both ADAMTS13 and TSP-1 antigen levels are significantly reduced in TTP plasma and hence the combined effect may be responsible for the presence of ultra-long VWF multimers in TTP plasma. Interestingly ADAMTS13 antigen level appears to be increased in SLE/or ITP parients, in contrast to previous reports that showed reduced activity, whereas no changes in TSP-1-levels were found in those patients.