INFLUENCE OF MACROPHAGE RESISTANCE GENE LSH ITY/BCG (CANDIDATE NRAMP)ON TOXOPLASMA-GONDII INFECTION IN MICE/

Functional studies have shown that the murine macrophage resistance ge ne Lsh/Ity/Bcg (candidate Nramp) regulates macrophage priming/activati on for antimicrobial activity via the tumour necrosis factor-alpha (TN F-alpha)-dependent production of reactive nitrogen intermediates. Sinc e Toxoplasma gondii also parasitizes macrophages, is a stimulator of e ndogenous TNF-alpha release, and is sensitive to nitric oxide-mediated killing in activated macrophages, studies were carried out using chro mosome 1 congenic mouse strains to determine whether Lsh influences T. gondii infection. Two interesting observations were made: (i) contrar y to expectation, mice carrying the Lsh-resistant allele died earlier over the acute phase of infection than Lsh-susceptible mice; and (ii) Lsh-resistant mice which survived this acute phase of infection showed lower brain cyst numbers than the Lsh-susceptible mice. Whilst the la tter occurred independently of route of inoculation (oral, intraperito neal, or subcutaneous), the former was influenced both by the route of inoculation and the genetic background on which the Lsh-resistant all ele had been isolated. Hence, following oral administration of 20 brai n cysts of the RRA strain of T. gondii, mice carrying the Lsh-resistan t allele on a B10 genetic background showed a significantly enhanced r ate of mortality over the acute (first 8-12 days) phase of infection t han BIO Lsh-susceptible mice. Although this acute phase of infection i n B10 background mice was accompanied by an increase in serum TNF-alph a levels in both Lsh-resistant and -susceptible mouse strains, early m ortality preceded the TNF-alpha peak, and administration of neutralizi ng rabbit anti-TNF-alpha did not significantly enhance survival. Hence , inflammatory mediators other than TNF-alpha appear to be responsible for the increased rate of acute mortality observed in resistant mice. Infection intraper itoneally led to delayed mortality in B10 mice, wi th the mean time to 50% mortality now being significantly longer in Ls h-resistant than in Lsh-susceptible mice. On a BALB genetic background , it was the i.p. route of infection which led to acute mortality and more rapid death in the Lsh-resistant strain. When a less virulent ino culum was used and mortality delayed, Lsh-susceptible mice died more r apidly, and i.p. administration of rabbit anti-TNF-alpha led to 100% m ortality between days 8 and 10 of infection in both susceptible and re sistant mouse strains, consistent with a crucial protective role for T NF-alpha during this phase of infection. Overall these results show th at the Lsh gene plays different modulating roles over the course of T. gondii infection depending on the potency of the parasite inoculum, t he route and kinetics of infection, and the genetic background of the congenic mouse strains upon which the resistant allele has been isolat ed. These results probably reflect pleiotropic effects of macrophage p roducts stimulated by differential priming/activation rather than the direct effect of macrophage toxoplasmacidal activity.