CD4 + T Cells Modulate Expansion and Survival but Not Functional Properties of Effector and Memory CD8 + T Cells Induced by Malaria Sporozoites Michael G. Overstreet ¤ , Yun-Chi Chen, Ian A. Cockburn, Sze-Wah Tse, Fidel Zavala* Department of Molecular Microbiology and Immunology, Malaria Research Institute, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland, United States of America Abstract CD4 + helper T cells are critical orchestrators of immune responses to infection and vaccination. During primary responses, naı ¨ve CD8 + T cells may need ‘‘CD4 help’’ for optimal development of memory populations. The immunological factors attributed to CD4 help depend on the context of immunization and vary depending on the priming system. In response to immunization with radiation-attenuated Plasmodium yoelii sporozoites, CD8 + T cells in BALB/c mice fail to generate large numbers of effector cells without help from CD4 + T cells – a defect not observed in most systems. Given this unique early dependence on CD4 help, we evaluated the effects of CD4 + cells on the development of functional properties of CD8 + T cells and on their ability to abolish infection. First, we determined that this effect was not mediated by CD4 + non-T cells and did not involve CD1d-restricted NKT cells. We found that CD8 + T cells induced by sporozoites without CD4 help formed memory populations severely reduced in magnitude that could not limit parasite development in the liver. The inability of these ‘‘helpless’’ memory T cells to protect is not a result of defects in effector function, as their capacity to produce cytokines and undergo cytotoxic degranulation was indistinguishable from control memory T cells. These data indicate that CD4 + T help may not be necessary to develop the functional attributes of CD8 + T cells; however they are crucial to ensure the survival of effector and memory cells induced in primary responses. Citation: Overstreet MG, Chen Y-C, Cockburn IA, Tse S-W, Zavala F (2011) CD4 + T Cells Modulate Expansion and Survival but Not Functional Properties of Effector and Memory CD8 + T Cells Induced by Malaria Sporozoites. PLoS ONE 6(1): e15948. doi:10.1371/journal.pone.0015948 Editor: Laurent Re ´nia, Singapore Immunology Network-A*STAR, Singapore Received September 25, 2010; Accepted November 30, 2010; Published January 4, 2011 Copyright: ß 2011 Overstreet et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by NIH grant number AI44375 to FZ (http://www.nih.gov). MGO and IAC were supported by fellowships from the Malaria Research Institute. The authors are grateful for the support of the Bloomberg Family Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: fzavala@jhsph.edu ¤ Current address: David H. Smith Center for Vaccine Biology and Immunology, University of Rochester Medical Center, Rochester, New York, United States of America Introduction CD8 + T cells are critical to protection against infection by intracellular pathogens, including liver stage malaria parasites. CD8 + T cells induced by immunization with radiation-attenuated Plasmodium sporozoites (c-spz) or sub-unit vaccines are capable of inhibiting the development of liver stage parasites [1,2,3,4]. T cell priming by c-spz occurs primarily in the skin-draining lymph node after parasite inoculation in the skin by either needle or the bite of an infected mosquito, followed by dissemination of effector T cells throughout the body, including the spleen and liver [4]. This priming in the lymph node is closely dependent on CD4 + cells and the absence thereof results in a reduced effector population [5]. This dependence on helper T cells at such an early time point is unique among models of CD8 + T cell priming, which often demonstrate unaltered primary CD8 + T cell responses to pathogens in the absence of helper T cells [6,7,8,9,10], with defects only apparent in functional recall of resting memory cells [7,9,11]. These studies have demonstrated an uncoupling of CD8 + T cell clonal expansion, survival, and acquisition of effector function. In view of the clear and early dependence of c-spz- induced CD8 + T cells on CD4 + T cells, we sought to characterize the effect of helper T cells on the functional development of anti- parasite CD8 + T cells. In the current study, we evaluated the role of CD4 + T cell help on the development of functional anti-malaria effector and memory CD8 + T cells by using Thy-1 allelic mismatched T cells so that survival could be measured independently of effector function. We found that while effector and memory CD8 + T cells from CD4-depleted mice (‘‘helpless’’ T cells) were severely reduced in magnitude compared to those primed in the presence of CD4 + T cells, helpless effector and memory CD8 + T cells were fully competent to produce cytokines and degranulate upon restimu- lation ex vivo. Interestingly, however, helpless CD8 + T cells failed to confer any level of protection against live sporozoite infection, indicating that large numbers of anti-parasite CD8 + T cells are critical to protection. Our studies indicate that the role of CD4 + T cells in modulating the CD8 + T cell response to the circumspor- ozoite protein of irradiated P. yoelii sporozoites appears to be restricted to ensuring the survival of activated T cells, without a discernible effect on the development of their functional properties. PLoS ONE | www.plosone.org 1 January 2011 | Volume 6 | Issue 1 | e15948