An ELISA for SGP28/CRISP-3, a cysteine-rich secretory protein in human neutrophils, plasma, and exocrine secretions

Specific granule protein of 28 kDa (SGP28), also termed cysteine-rich secretory protein 3 (CRISP-3), is a glycoprotein that belongs to a family of cysteine-rich secretory proteins (CRISPs). SGP28 was originally discovered in human neutrophils, but transcripts are widely distributed in exocrine glands (salivary glands, pancreas, and prostate) and also found at lower levels in epididymis, ovary, thymus, and colon. The function of SGP28/CRISP-3 is not yet known. Similarities to pathogenesis-related proteins in plants and the expression in neutrophils and exocrine glands suggest that SGP28/CRISP-3 may play a role in innate host defense. We describe here the production of a recombinant, C-terminally truncated form of CRISP-3 (rCRISP-3Δ) and the generation of polyclonal antibodies against rCRISP-3Δ that are useful in immunoblotting and immunocytochemistry. We present a specific, accurate, and reproducible enzyme-linked immunosorbant assay (ELISA) for the measurement of CRISP-3 with a detection limit of 2 ng/ml. We further demonstrate the presence of CRISP-3 protein in human plasma (6.3 μg/ml), saliva (21.8 μg/ml), seminal plasma (11.2 μg/ml), and sweat (0.15 μg/ml), and describe the coexistence of two different molecular weight forms of CRISP-3, representing an N-glycosylated and a non-glycosylated form of the mature protein.