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

Assuming that haptoglobin, by virtue of its immunomodulatory properties, could be a regulatory factor during reproduction, its presence in the human uterus was determined. Protein extracts from endometrial tissue samples of pregnant and non-pregnant women were analysed by the immunoblot technique and 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 by Western blot analysis for its haptoglobin identity, was used for amino-terminal sequencing. Sequencing of the 42 kDa protein identified it as the beta chain of haptoglobin. Immunohistochemistry was used to corroborate the findings and to visualize the distribution of haptoglobin in the tissue. The intensity of the 42 kDa band derived from decidua graviditatis was significantly higher than the intensity of bands derived from non-pregnant endometrium in the proliferative phase (P < 0.01) and in the secretory phase (P < 0.05). Immunohistochemical staining with anti-human haptoglobin antibody elicited strong signals in the decidua graviditatis and weaker signals in the normal endometrium, with the latter showing menstrual cycle-dependent variation. Moderate staining of stroma and a lack of staining of epithelium in the proliferative phase contrasted with the strong staining of stroma and moderate level of staining of epithelium observed in the secretory phase. Haptoglobin in the uterus may exert several functions such as the known binding of haemoglobin, but could also be involved in the multi-factorial mechanism protecting the fetus from a maternal allograft-like immune response.