Improved P. falciparum Mouse Model Figure 5. Examples of parasitemia obtained in the NSG-IV model. Panels (A) and (B) illustrate the typical predominance of ring stage parasites seen at 48 h intervals (see Fig. 4 C and SI-II), with frequent polyparasitism. Such parasite densities are common using UPA parasites in mice having .75% chimerism. Panel (C) fully mature trophozoites seen 24 hours later, as compared to A and B. Single nucleus trophozoites predominate, though some can be mistaken for schizonts due to polyparasitism. However rings and young schizonts are also present in small proportions. Panel (D) illustrates one of the highest parasitemias obtained. Insert: active phagocytosis by a circulating monocyte. Panels (E) and (F) illustrate fully mature schizonts which are only occasionally seen in peripheral blood. Panel (G) stage IV gametocyte, Panel (H) rare occurrence of mature gametocyte (stage V) in peripheral blood. doi:10.1371/journal.pone.0018045.g005 liposomal-clodronate for macrophage suppression, and the IL-2Rc mutation on the NOD/SCID genetic background, we have achieved a model that has 100% reproducibility, is receptive to numerous strains without requiring pre-adaptation of the parasite, and supports fast-rising, high-reaching P. falciparum parasitemia that remains stable for weeks or months. Results also show that the pro-inflammatory effect of P. falciparum is better controlled, however still suboptimally. In addition to clodronate encapsulated liposomes (clo-lip) [6], we now demonstrate, in NOD/SCID mice, the immunomodulatory effects of aging, of inosine and of the IL-2 receptor c mutation. The latter brings the greatest breakthrough with a marked reduction in inflammatory cytokines/chemokines and, most likely for this reason, improved P. falciparum survival. Results are in keeping with those reported recently using an hybrid IP-IV model and an immunodeficient mouse-adapted P. falciparum strain [14]. The combination of an IV-IV protocol and prudent use of immunosuppressive factors has produced a model that has a number of attractive features over those previously described. The main advantage is an extremely high success rate, with 100% of mice that were infected showing a healthy P. falciparum parasitemia. This represents a dramatic improvement compared to the low proportion of mice with stable parasitemia that we obtained in previous models [6,8]. By using the IV route and clo- lip we reduced huRBC requirements and above all excluded the unpredictable, poorly understood transperitoneal passage of huRBCs as employed in existing models [6,11,21]. Another advantage is the high reproducibility. The parasitemia curves were very consistent from one animal to the other with low standard deviations, a feature that will be important when using the model for vaccine or drug studies. This level of consistency is, in fact, more so than what is seen in human infections where untreated cases show day-to-day variations in parasitemia by one degree of magnitude or more [22]. The parasitemia is also capable of reaching high levels of density, particularly when calculated as a percentage of the huRBC subset rather than total erythrocytes. These favourable characteristics of the model are complemented by a rapid rate of parasite growth, which rises faster than in other models [6,9,21], with very healthy looking parasites and frequent polyparasitism of huRBC. Several results concur to suggest that the model is closer to events recorded in humans than other existing P. falciparum models, namely receptivity to non-adapted parasites, synchronisation, partial sequestration of mature forms and gametocyte production. All P. falciparum strains tested so far have been supported, indicating that, in contrast to Aotus, Saimiri and previous mouse models, there is no requirement for preliminary adaptation of the parasite to this new host [11,14,23,24,25]. This was true for the 3 strains employed and for one clinical isolate of P. falciparum tested. Results obtained with the latter bode well for the potential of the model to support P. vivax growth. This is the only Malaria model that has ever demonstrated the phenomenon of synchronisation, which is not seen either in Aotus, Saimiri, or other immunodeficient mice [11,14,23,24,25]. Exam- PLoS ONE | www.plosone.org 8 March 2011 | Volume 6 | Issue 3 | e18045