Protein S Free Antigen Levels and the Occurrence of IgG and IgM Antibodies to Protein S in Normal Pregnancy.
Authors: Arkel Yale S. , Ku De-Hui W. , Paidas Michael J. , Fortunato Christine , Ku Evelyn , Lockwood Charles J. 
Published: October 2019

The evaluation of thrombophilia (TP) in pregnancy is central to the assessment for the risk for adverse pregnancy outcomes (APO) and thrombosis. Antibodies to protein S (PS) have been described in patients with autoimmune diseases and have also been related to thrombosis and APO in pregnancy. Therefore several major issues exist in the evaluation of PS in pregnancy for TP and the risk for APO and thrombosis. What are the ranges of free PS antigen (FPSAG) levels in normal pregnancy (NP)? Do antibodies to PS occur in so called NP and do they affect the levels of PS? We studied 82 1st trimester (TRI) samples, 64 2nd TRI samples, and 78 3rd TRI samples from normal pregnancies to evaluate the FPSAG levels and antibodies against PS both IgG and IgM. We also tested 50 normal non-pregnant donors for antibodies against PS to establish the non-pregnant reference range. The data are show in table 1. It reveals that even in NP the FPSAG levels can decrease to as low as 28% in 1st and 2nd TRI, and 20% in 3rd TRI. These values are significantly below the non-pregnant values. The differences of decreased of PS between 1st vs 3rd, and 2nd vs 3rd TRI are statistically significant (p<0.05). We have previously reported that PS lower than 30% in the 2nd to 3rd TRI was associated with a higher risk for APO. If additional TP factors are present, PS levels in lower NP reference range may increase the risk for APO and thrombosis. The data also shows that there is an increase in PS IgG and IgM antibodies during gestation compared to non-pregnant controls. The number of patients with IgG and IgM antibodies above the 3SD of control value increase with gestation (3% vs 8% vs 9% for IgG, and 13% vs 26% vs 31% for IgM, [1st TRI, 2nd TRI, 3rd TRI respectively]). The lack of overall correlation of antibodies with the protein level is similar to the finding of Gris et al with protein Z and APO. Do the antibodies affect the function but not the level of the factors responsible for the natural inhibition of the prothrombotic process? Do these antibodies become part of the pathogenesis of APO and thrombosis in pregnancy associated with the anticardiolipin syndrome? These questions need to be addressed by larger studies of normal and adverse pregnancy patients. The increase in the number of NP subjects with high levels of PS-IgG and IgM antibodies with gestation might indicate an enhanced immune response to components of the hemostatic system in some patients who might become more vulnerable to APO. The low values of FPSAG in NP add to the difficulty of assessing for a pre-existing deficiency state. However, if an inherited deficiency is present the FPSAG should be in the lower ranges of those we have established for the three TRI. This need to be defined by the assessment of a larger group of PS deficiency patients during pregnancy.

Table 1 shows the mean and standrad deviation of FPSAG levels and the titer of PS IgG and IgM antibodies

1st trimester 2nd trimester 3dr trimester CTR CTR= non-pregnant control FPSAG 38.8+/−10.5% 35.9+/−8.3% 27.9+/−7.0% 88+/−19% Anti-PS IgG 0.29+/−0.35OD 0.45+/−0.25OD 0.44+/−0.31OD 0.27+/−0.20OD Anti-PS IgM 0.48+/−0.38OD 0.66+/−0.42OD 0.61+/−0.36OD 0.26+/−0.15OD