Minor histocompatibility antigens are expressed in syncytiotrophoblast and trophoblast debris: implications for maternal alloreactivity to the fetus

Abstract

The fetal semi-allograft can induce expansion and tolerance of antigen-specific maternal T and B cells through paternally inherited major histocompatibility complex and minor histocompatibility antigens (mHAgs). The effects of these antigens have important consequences on the maternal immune system both during and long after pregnancy. Herein, we investigate the possibility that the placental syncytiotrophoblast and deported trophoblastic debris serve as sources of fetal mHAgs. We mapped the expression of four mHAgs (human mHAg 1, pumilio domain-containing protein KIAA0020, B-cell lymphoma 2-related protein A1, and ribosomal protein S4, Y linked) in the placenta. Each of these proteins was expressed in several placental cell types, including the syncytiotrophoblast. These antigens and two additional Y chromosome-encoded antigens [DEAD box polypeptide 3, Y linked (DDX3Y), and lysine demethylase5D] were also identified by RT-PCR in the placenta, purified trophoblast cells, and cord blood cells. Finally, we used a proteomic approach to investigate the presence of mHAgs in the syncytiotrophoblast and trophoblast debris shed from first-trimester placenta. By this method, four antigens (DDX3Y; ribosomal protein S4, Y linked; solute carrier 1A5; and signal sequence receptor 1) were found in the syncytiotrophoblast, and one antigen (DDX3Y) was found in shed trophoblast debris. The finding of mHAgs in the placenta and in trophoblast debris provides the first direct evidence that fetal antigens are present in debris shed from the human placenta. The data, thus, suggest a mechanism by which the maternal immune system is exposed to fetal alloantigens, possibly explaining the relationship between parity and graft-versus-host disease.

Figure 1

Minor antigen mRNA expression in placenta, cytotrophoblast cells, and fetal cord blood. A: Real-time RT-PCR for autosomally encoded mHAgs (HMHA1, KIAA0020, and BCL2A1) and the Y chromosome–encoded mHAg (KDM5D), for placental lysate and purified cell populations. Data are expressed as the mean ± SEM difference in C_T values between the gene of interest and β-actin for the same samples, calculated as described in Materials and Methods. The average ± SEM β-actin values in each sample were as follows: 24.9 ± 0.59 (placenta), 21.4 ± 0.24 (cytotrophoblast), and 26.53 ± 1.56 (cord blood mononuclear cells). B and C: Conventional RT-PCR data for placental lysate and purified cells, respectively. Glyceraldehyde 3-phosphate dehydrogenase (G3PDH) was used as a control; the sex of the infant is indicated by symbols above each lane. CB, cord blood mononuclear cells; CT, cytotrophoblast cells; M, mol. wt. marker.

Figure 2

mHAg expression in first- and second-trimester placentas. A, C, E, G, and I: First-trimester placenta (gestational age, 8 to 12 weeks). B, D, F, H, and J: Second-trimester placenta (gestational age, 13 to 21 weeks). Reddish-brown staining represents specific immunoreactivity of mHAg-specific antibodies with cells; bluish-purple staining, hematoxylin counterstain. Arrowheads indicate syncytiotrophoblast; arrows, Hofbauer cells. IVS, intervillous space; CC, trophoblast cell column.

Figure 3

mHAg expression in term placentas and extraplacental membranes. A, C, E, G, and I: Term placenta. B, D, F, H, and J: Extraplacental membranes. Arrowhead indicates syncytiotrophoblast; arrow, Hofbauer cells. A, amnion membrane; Ch, chorion membrane; Dec, decidua; IVS, intervillous space.