Postnatal development of resistance against infection in an experimental model.

The postnatal development of resistance against infection was monitored by the treatment of juvenile mice with a virulent strain of Listeria monocytogenes. It could be shown that until day 10 after birth, young mice succumbed to an infection with even minimal doses of bacteria. Between day 15 and 30, the resistance against infection gradually increases until the rather constant level of grown-up animals is reached (Fig. 1). Juvenile mice that survive the primary infection are able to build up a state of immunity, which is rather similar to that of grown-up mice (Fig 3). Immunity against L. monocytogenes is mainly expressed by a functionally active T-cell system; the maturity of these cells in 15 days old mice could be demonstrated by the transfer of cells to "nude"-mice, which lack a thymus (Fig. 4). A significant increase of the non-specific resistance can be achieved even in 10 days old mice by the injection of adjuvants like pertussis organisms or endotoxin of Salmonella typhi some days before infection (Fig. 5, Fig. 6). Our findings suggest that a deficiency of functionally active macrophages is responsible for the insufficient resistance against infection with L. monocytogenes in young mice.