Haptoglobin is present in human endometrium and shows elevated levels in the decidua during pregnancy

Assuming that haptoglobin, by virtue of its immunomodulatory properties, co uld be a regulatory factor during reproduction, its presence in the human u terus was determined. Protein extracts from endometrial tissue samples of p regnant and non-pregnant women were analysed by the immunoblot technique an d the intensities of specific bands were quantified. Bands corresponding to haptoglobin were identified in tissue samples obtained from both sources. Protein, purified by high-performance liquid chromatography and monitored b y Western blot analysis for its haptoglobin identity, was used for amino-te rminal sequencing. Sequencing of the 42 kDa protein identified it as the be ta chain of haptoglobin. Immunohistochemistry was used to corroborate the f indings and to visualize the distribution of haptoglobin in the tissue. The intensity of the 42 kDa band derived from decidua graviditatis was signifi cantly higher than the intensity of bands derived from non-pregnant endomet rium in the proliferative phase (P < 0.01) and in the secretory phase (P < 0.05). Immunohistochemical staining with anti-human haptoglobin antibody el icited strong signals in the decidua graviditatis and weaker signals in the normal endometrium, with the latter showing menstrual cycle-dependent vari ation. Moderate staining of stroma and a lack of staining of epithelium in the proliferative phase contrasted with the strong staining of stroma and m oderate level of staining of epithelium observed in the secretory phase. Ha ptoglobin in the uterus may exert several functions such as the known bindi ng of haemoglobin, but could also be involved in the multi-factorial mechan ism protecting the fetus from a maternal allograft-like immune response.