Binds to the small latent transforming growth factor-beta complex, consisting of the N-terminal TGFB1 latency-associated peptide (LAP) and the mature form of TGFB1, thereby leading to the activation of TGFB1 (PubMed:27389696). The activation of TGFB1 leads to stimulation of naive CD4(+) T-cells to increase FoxP3 expression and to an increase in the number of FoxP3(+) regulatory T-cells (PubMed:27389696). Induces the differentiation of a suppressive CD4(+)LAP(+)FoxP3(-) T-cell subset (PubMed:27389696). Induces the secretion of TGFB1 in macrophages, but not in activated CD4(+) T-cells (PubMed:27389696). May reduce the expression of several pro-inflammatory cytokines and chemokines by CD4(+) T-cells, including IL2 and IL6 (PubMed:27389696). Interacts with latency-associated peptide; leading to TGFB1 activation. PSBG are produced in high quantity during pregnancy. Belongs to the immunoglobulin superfamily. CEA family.