Toivo Hirvonen

Development of Mitogen Responding T Cells and Natural Killer Cells in the Human Fetus

Human T lymphocytes acquire immunocompetence during fetal life, even though at the time of full-term birth their functional capacity may not be equal to that ofadult lymphocytes (Toivanen et al. 1978, Stiehm et al. 1979, Stites & Pavia 1979, Toivanen & Granberg 1980). In this paper we will summarize and discuss recent studies carried out in our laboratory on the fetal development of human T cells responding to mitogens and of cytotoxic cells, paying particular attention to the appearance of natural killer (NK) cell activity.

Cell-mediated lympholysis by fetal and neonatal lymphocytes in sheep and man

Abstract The occurrence of cell-mediated lympholysis (CML) was studied in the fetal lamb at 70 to 138 days of gestation and in the human fetus at 12 to 23 weeks of gestation, and also at the time of full-term birth in both species. The fetal lymphocytes were sensitized in mixed-lymphocyte culture (MLC) to prepare effector cells for CML. Concanavalin A-induced cells were used as target cells. No fetal capacity for CML was demonstrable during those periods of intrauterine life studied, despite the occurrence of clear MLC responses. At the time of full-term birth. CML by lamb lymphocytes was on average only one-seventh of the adult response. In man, neonatal lymphocytes may occasionally show a CML of the same magnitude as the adult lymphocytes, although on average the neonatal CML capacity was about half that in the adult.

Development of T Cell Repertoire in the Human and the Sheep Fetus

In the thymus, precursor T cells acquire the capacity to recognize structures determined by the individuals own major histocompatibility complex (MHC) genes. Only thereafter are mature T cells capable of cooperating with other compatible lymphocytes or macrophages and of generating MHC-restricted cytotoxicity (Zinkernagel et al., 1978a, b). These functions are of vital importance for all higher forms fo life. However, during evolution, recognition and destruction of foreign cells may have been of even greater importance, and in more developed animals, including man, immunological defence against foreign antigens is always significant. In this paper we will summarize and discuss recent studies carried out in our laboratory on fetal development of the T cell repertoire responsible for recognition and destruction of foreign cells. In these studies with cells of human and sheep fetuses, mixed lymphocyte cultures (MLC) and cell mediated lympholysis (CML) have been used. To determine the specificity of the MLC-responses observed, we have applied the cell suicide technique with 5-bromodeoxyuridine (BudR) and UV light (Zoschke & Bach, 1971, Asantila et al., 1974a, Asantila & Toivanen, 1976). CML-responses have been determined by using *̂Cr release and specifically sensitized effector cells (Hayry & Defendi 1970, Granberg et al., 1976a).

Ontogeny of phytohemagglutinin and concanavalin A responses in the human fetus: Effect of thymosin

Abstract Ontogenic development of phytohemagglutinin (PHA) and concanavalin A (Con A) responses of cells from 22 human fetuses at 6–23 weeks of gestation was studied. The results show that PHA responses appear earlier and stronger than Con A responses during the early fetal period. Similar to the findings reported for the mouse, sheep, and guinea pig, the mitogen responsive cells were first found in the thymus and thereafter in the spleen. The effect of thymosin on human fetal cells was variable, since thymocytes and spleen cells of only 2 out of 13 fetuses studied were susceptible to low concentrations (0.001–0.1 μg/ml) of thymosin. Higher concentrations (1.0–200 μg/ml) increased 5[125I]iodo-2′-deoxyuridine (125IUdR) uptake of PHA- and Con A-stimulated cells in few fetuses. In the fetal liver and bone marrow, thymosin-induced increase of 125IUdR uptake was observed both in unstimulated and mitogen-stimulated cells, however, without any significant effect on stimulation indices.

Alpha-naphthyl acetate esterase activity in human foetal lymphocytes

Human foetal mononuclear cells from thymus, spleen, liver, bone marrow and peripheral blood at 8-24 weeks of gestation were examined for cytochemical evidence of acid alpha-naphthyl acetate esterase (ANAE) activity. The focal brownish-red ANAE reaction product (T cell staining pattern) was observed in counterstained cytocentrifuged cell smears in the cytoplasm. ANAE-positive lymphoid cells were first observed in the thymus at 9 weeks of gestation. A gradual increase in frequency of ANAE-positive cells in foetal thymus was observed, from about 10% at 14-15 weeks to about 20% at 22-24 weeks of gestation. By 14 weeks of foetal age, spleen and liver contained a few ANAE-positive cells and after 15 weeks of gestation consistent occurrence of ANAE-positive cells was observed in foetal bone marrow and peripheral blood. These results demonstrate that ANAE-positive lymphocytes first appear in the foetal thymus and are subsequently found in the foetal liver, spleen, bone marrow and peripheral blood.

Inhibition by anti-β2-microglobulin of MLR by human fetal liver cells

Abstract Liver cells of fetuses without lymphoid thymus show strong, immunologically nonspecific proliferation when confronted with adult foreign lymphocytes. To elucidate the mechanism of this response, we have studied the effect of anti-β2-microglobulin on fetal liver cells in mixed lymphocyte reaction (MLR). Anti-β2-microglobulin inhibited MLR when antiserum was present in the culture or when the responding fetal cells had been preincubated with it. If the adult lymphocytes stimulating fetal liver cells were preincubated with anti-β2-microglobulin, only a slight or no effect was observed. No difference was observed between liver cells from fetuses with and without lymphoid thymus. β2-microglobulin was also demonstrated on the fetal liver cells by immunofluorescence. The results reveal that β2-microglobulin is present on early fetal liver cells, and that MLR by fetal liver cells is inhibited by anti-β2-microglobulin in the same way as in adults. multipotential immunocompetence of early fetal liver cells cannot be excluded on the basis of these experiments.