Selectively Impaired CD8  but Not CD4  T Cell Cycle Arrest during Priming as a Consequence of Dendritic Cell Interaction with Plasmodium-Infected Red Cells 1 Dodie S. Pouniotis, Owen Proudfoot, Violeta Bogdanoska, 3 Karen Scalzo, Svetozar Kovacevic, 4 Ross L. Coppel, 4 and Magdalena Plebanski 2 Individuals living in malaria-endemic areas show generally low T cell responses to malaria Ags. In this study, we show murine dendritic cell (DC) interaction with parasitized erythrocytes (pRBC) arrested their maturation, resulting in impaired ability to stimulate naive, but not recall T cell responses in vitro and in vivo. Moreover, within the naive T cell population, pRBC-treated DC were selectively deficient in priming CD8  but not CD4  T cells. Indeed, DC that had taken up pRBC were shown for the first time to efficiently prime CD4  T cell responses to a known protective merozoite Ag, MSP4/5. In contrast, impaired priming resulted in decreases in both proliferation and cytokine production by CD8  T cells. Deficient priming was observed to both a model and a Plasmodium berghei-specific CD8  T cell epitope. The mechanisms underlying the inability of parasite-treated DC to prime CD8  T cells were explored. pRBC treatment of DC from wild-type C57BL/6, but not from IL-10 knockout animals, suppressed DC-mediated T cell priming across a Transwell, suggesting active IL-10-dependent suppression. CD8  T cells were arrested at the G 0 stage of the cell cycle after two cell divisions post-Ag stimulation. The proliferation arrest was partially reversible by the addition of IL-2 or IL-7 to responder cultures. These results suggest that in malaria-endemic areas, priming of CD8  T cell responses may be more difficult to induce via vaccination than the priming of CD4  T cells. Moreover, pathogens may selectively target the CD8  T cell arm of protective immunity for immune evasion. The Journal of Immunology, 2005, 175: 3525–3533. M alaria remains the most important tropical disease af- flicting humans, with recent mortality and incidence estimates of 2–3 million deaths and 300 –500 million clinical cases annually (1). A better understanding of the immune response to the malaria parasite and the nature and regulation of protective mechanisms would facilitate the development of a much needed vaccine. A number of mechanisms have been proposed to explain how the asexual erythrocytic parasites survive in the face of the host’s immune response, including antigenic diversity, clonal Ag variation, and altered peptide ligand antagonism (re- viewed in Ref. 2). Low numbers of Plasmodium falciparum-specific memory T cells, particularly CD8 + T cells, have been observed in individuals living in malaria-endemic areas (3–7). Impaired APC function as a result of exposure to parasitized erythrocytes (pRBC) 5 may con- tribute to this observation. Indeed, Urban et al. reported that ex- posure of human dendritic cells (DC) to pRBC promoted a matu- ration defect, and although the consequences of this interaction for immunity were not assessed in vivo, a subsequent impairment in T cell priming was apparent in vitro (8). As well as recent reports of pRBC-induced impairment of DC maturation (9), there are reports suggesting the opposite: pRBC- induced DC activation and up-regulation of costimulatory mole- cules (10). Results from the present study supported and extended the former finding, showing a reproducible DC-maturation defect in- duced by both Plasmodium yoelii nigeriensis lethal (P. yoelii) or Plas- modium chabaudi chabaudi (P. chabaudi) pRBC. Further investigat- ing this observation, the ability of pRBC-exposed DC to stimulate CD4 + as compared with CD8 + T cells was determined, as was their ability to prime recall compared with nonrecall T cell responses. Our results suggest that although maturation-arrested DC have a specific defect in priming of naive CD8 + T cells, other important functions can remain largely unaffected. The mechanism of this impaired immunity was found to be novel, with active IL-10-me- diated suppression of proliferation after two CD8 + T cell divisions resulting in G 0 -arrested T cells. The arrest was partly reversible by the addition of IL-2 or IL-7. Materials and Methods Mice C57BL/6 (H-2K b ), BALB/c (H-2K d ), and IL-10BL/6KO (H-2K b ) female mice, 6 – 8 wk old were used in the experiments. Mice were bred at the Vaccine and Infectious Diseases Unit, The Austin Research Institute, Heidelberg, Victoria, Australia Received for publication June 24, 2004. Accepted for publication June 7, 2005. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 D.S.P. was supported by National Health and Medical Research Council Grant 223302. O.P. was supported by a LaTrobe University Doctor of Philosophy Schol- arship. M.P. and R.L.C., Howard Hughes Medical Institute International Scholars, are supported by National Health and Medical Research Council Program Grant 334012. M.P. is a National Health and Medical Research Council Senior Fellow. 2 Address correspondence and reprint requests to Dr. Magdalena Plebanski, Vaccine and Infectious Diseases Unit, The Austin Research Institute, Austin Campus, Hei- delberg, Victoria 3084, Australia. E-mail address: mplebans@ari.unimelb.edu.au 3 Current address: Clinical Research Center for Chronic Inflammatory Diseases, Uni- versity of Melbourne, Department of Medicine, Royal Melbourne Hospital, Clinical Sciences Building, Royal Parade, Parkville, Victoria 3050, Australia. 4 Current address: Department of Microbiology, Monash University, Victoria 3800, Australia. 5 Abbreviations used in this paper: pRBC, parasitized erythrocyte; DC, dendritic cell; P. yoelii, Plasmodium yoelii nigeriensis lethal; P. chabaudi, Plasmodium chabaudi chabaudi; AP, alkaline phosphatase; SFU, spot-forming units; PI, propidium iodide; MFI, mean fluorescence intensity; KO, knockout. The Journal of Immunology Copyright © 2005 by The American Association of Immunologists, Inc. 0022-1767/05/$02.00