Ontogeny of mitogenic responsiveness to PHA and sheep erythrocytes and lymphokine production in foetal guinea-pigs.

Pre-culture of spleen cells for 24 h before addition of activator greatly improved the mitogenic response to phytohaemagglutinin (PHA) and sheep erythrocytes (SRBC). Spleen cells were separated into plastic-adherent and non-adherent populations and in some experiments purified further to 'macrophages' and 'lymphocytes' respectively. A critical mixture of lymphocytes and macrophages (10:1) gave twice the mitogenic response of lymphocytes alone to PHA and SRBC in cultures of foetal, juvenile and adult guinea-pig spleen cells. Vigorous mitogenic responses to PHA and SRBC were found at 36 days of gestation. The mitogenic response increased from 46-56 days of gestation to above adult level. Lipopolysaccharide (LPS) obtained by the Westphal method was a superior mitogen to LPS extracted by the Boivin technique. Plaque-forming cells (PFC) could not be induced in vitro at any age even with the addition of LPS. Evidence was obtained for mitogenic and blastogenic lymphokines produced by lymphocytes activated with PHA or produced by lymphocytes activated with PHA or SRBC (but not LPS). These lymphokines were produced by activated splenic lymphocytes of a 40-day-old foetus; older animals showed no evidence for a quantitative increase in lymphokine production. The onset and maturation of mitogenic responsiveness in the guinea-pig and human foetus is compared by age-equivalence.