Transforming Growth Factor β Production Is Inversely Correlated with Severity of Murine Malaria Infection

We have examined the role of the immunomodulatory cytokine transforming growth factor (TGF)-β in the resolution and pathology of malaria in BALB/c mice. Circulating levels of TGF-β, and production of bioactive TGF-β by splenocytes, were found to be low in lethal infections with Plasmodium berghei. In contrast, resolving infections with P. chabaudi chabaudi or P. yoelii were accompanied by significant TGF-β production. A causal association between the failure to produce TGF-β and the severity of malaria infection was demonstrated by treatment of infected mice with neutralizing antibody to TGF-β, which exacerbated the virulence of P. berghei and transformed a resolving P. chabaudi chabaudi infection into a lethal infection, but had little effect on the course of P. yoelii infection. Parasitemia increased more rapidly in anti–TGF-β–treated mice but this did not seem to be the explanation for the increased pathology of infection as peak parasitemias were unchanged. Treatment of P. berghei–infected mice with recombinant TGF-β (rTGF-β) slowed the rate of parasite proliferation and prolonged their survival from 15 to up to 35 d. rTGF-β treatment was accompanied by a significant decrease in serum tumor necrosis factor α and an increase in interleukin 10. Finally, we present evidence that differences in TGF-β responses in different malaria infections are due to intrinsic differences between species of malaria parasites in their ability to induce production of TGF-β. Thus, TGF-β seems to induce protective immune responses, leading to slower parasite growth, early in infection, and, subsequently, appears to downregulate pathogenic responses late in infection. This duality of effect makes TGF-β a prime candidate for a major immunomodulatory cytokine associated with successful control of malaria infection.