Differences in susceptibility of various mouse strains to haemoprotozoan infections: possible correlation with natural killer activity.

Striking differences in the susceptibility to P. chabaudi and B. microti infections among different strains of mice are described. In most of the strains, both parasites induce a moderate, transient parasitaemia, followed by recovery. However, in the A strain P. chabaudi is rapidly lethal and B. microti induces a parasitaemia which persists for life in most animals. In contrast B10.A mice, which have the same H-2q haplotype as the A strain, recover from both infections. In first crosses (A X B10.A)F1 mice are uniformly resistant. Among other correlates, NK cells activity may be relevant. This activity is known to be high in strain of mice which are resistant to both of the parasites studied, such as C57B1 and CBA mice, and low in the susceptible A strain. We have examined the spleen weight, total cell number and NK activity at different times after infection. In the resistant strains there is a rapid increase in size and cell number which is still more marked during recovery. NK activity increases greatly during the infection; this is especially remarkable considering the dilution of nucleated cells with erythrocytes precursors. In contrast, in A mice cell numbers of NK activity remain almost unchanged. Thus, marked activation of NK cells occur, in resistant strains but not in susceptible ones. Possible mechanisms of activation of these cells and their effect in haemoprotozoan infections are discussed.