Ki-A10, a germ cell nuclear antigen retained in a subset of germ cell-derived tumors

Abstract

Monoclonal antibody Ki-A10 recognizes a nuclear antigen of 25 and 22 kd apparent molecular mass, which is abundantly expressed by immature gonocytes, spermatogonia, and spermatocytes, whereas it is absent in spermatids, spermatozoa, oocytes, and normal somatic tissues. In a broad spectrum of human cancers the antibody showed no reactivity except for a small subset of malignant lymphomas. Because of this restricted expression pattern, we examined 173 germ cell tumors and 18 sex cord stromal tumors immunohistochemically to assess the distribution of the Ki-A10 antigen. A strongly positive reaction was found in classic seminomas, dysgerminomas, spermatocytic seminomas, and the germ cell component of gonadoblastomas. Yolk sac tumors presented a heterogeneous reactivity pattern ranging from overall positivity to complete lack of antigen expression, and in three of eight choriocarcinomas, a few clusters of cytotrophoblast cells were strongly labeled. All other tumors, including Leydig and Sertoli cell tumors as well as placental tissue, were negative. Our findings suggest that specific germ cell antigens can be retained in germ cell tumors along particular differentiation pathways. Ki-A10 is the first marker that consistently labels spermatocytic seminoma, further confirming its germ cell origin and suggesting a close relationship to classic seminoma. The antibody may serve for diagnostic purposes and promises new insights into the process of germ cell differentiation and the development of germ cell-derived neoplasia.

Figure 1.

a: Section of a normal adult testis stained with Ki-A10; spermatogonia are consistently positive (red label); the staining intensity decreases in spermatocytes along with maturation and becomes undetectable in spermatids and spermatozoa. Sertoli cells show no reactivity (original magnification, ×350). b: Strongly Ki-A10-labeled tumor cell nuclei of testicular seminoma scattered in a lymphoid infiltrate negative to Ki-A10 (magnification, ×140). c: The totality of the bizarre cells of spermatocytic seminoma exhibit strong nuclear Ki-A10 reactivity (magnification, ×350). d: The germ cell component of gonadoblastoma stains with slightly variable intensity (brown label) for Ki-A10 (magnification, ×140). e: A large number of cells in YST of glandular type express the Ki-A10 antigen; focally, abrupt loss of antigenity is noted (upper right) (magnification, ×140). f: Embryonal carcinoma characteristically does not express the Ki-A10 antigen; in the preserved seminiferous tubules (bottom right), spermatogonia and spermatocytes are strongly positive (magnification, ×140; all sections peroxidase technique and hematoxylin counterstain).

Figure 2.

Immunoprecipitation of nuclear extracts of L428 cells with Ki-A10 at 4°C (left) and room temperature (right) showing two neat protein bands corresponding to approximately 25 and 22 kd and a faint band near 46 kd. The signal intensity was slightly weaker when incubation was performed at room temperature. The thick bands at approximately 180 to 200 kd represent the nonreduced antibody Ki-A10 with the molecular mass of an IgG molecule. Molecular weight reference is indicated on the right.

Figure 3.

Western blot experiment using lysates from normal testicular tissue and seminoma. Both lanes show a strong signal at approximately 25 kd; a second band of lesser intensity is seen slightly above 46 kd, which corresponds to the upper band in Figure 2 ▶ . The apparent difference in molecular mass is probably because of slight variations in the migration of the molecular weight reference (indicated on the left).