IFN-γ and IL-21 Double Producing T Cells Are Bcl6-Independent and Survive into the Memory Phase in Plasmodium chabaudi Infection

Victor H Carpio,Robin Stephens,Pinaki P Banerjee,Michael M Opata,Alexander L Dent,Marelle E Montañez,Moriya Tsuji

doi:10.1371/journal.pone.0144654

Abstract

CD4 T cells are required to fight malaria infection by promoting both phagocytic activity and B cell responses for parasite clearance. In Plasmodium chabaudi infection, one specific CD4 T cell subset generates anti-parasitic IFN-γ and the antibody-promoting cytokine, IL-21. To determine the lineage of these multifunctional T cells, we followed IFN-γ+ effector T cells (Teff) into the memory phase using Ifng-reporter mice. While Ifng + Teff expanded, the level of the Th1 lineage-determining transcription factor T-bet only peaked briefly. Ifng + Teff also co-express ICOS, the B cell area homing molecule CXCR5, and other Tfh lineage-associated molecules including Bcl6, the transcription factor required for germinal center (GC) T follicular helper cells (Tfh) differentiation. Because Bcl6 and T-bet co-localize to the nucleus of Ifng + Teff, we hypothesized that Bcl6 controls the Tfh-like phenotype of Ifng + Teff cells in P. chabaudi infection. We first transferred Bcl6-deficient T cells into wildtype hosts. Bcl6-deficient T cells did not develop into GC Tfh, but they still generated CXCR5+IFN-γ+IL-21+IL-10+ Teff, suggesting that this predominant population is not of the Tfh-lineage. IL-10 deficient mice, which have increased IFN-γ and T-bet expression, demonstrated expansion of both IFN-γ+IL-21+CXCR5+ cells and IFN-γ+ GC Tfh cells, suggesting a Th1 lineage for the former. In the memory phase, all Ifng + T cells produced IL-21, but only a small percentage of highly proliferative Ifng + T cells maintained a T-bethi phenotype. In chronic malaria infection, serum IFN-γ correlates with increased protection, and our observation suggests Ifng + T cells are maintained by cellular division. In summary, we found that Ifng + T cells are not strictly Tfh derived during malaria infection. T cells provide the host with a survival advantage when facing this well-equipped pathogen, therefore, understanding the lineage of pivotal T cell players will aid in the rational design of an effective malaria vaccine.

Highlights

Immunity against intracellular pathogens, such as the blood-stages of the rodent malaria parasite Plasmodium chabaudi, requires both antibodies and Th1-type responses [1, 2]
Similar expansion kinetics were observed for all Teff and Ifng/Thy1.1+ effector T cells along with parasite levels, which all peaked at day 9 p.i. (Fig 1C)
In order to investigate the degree of Th1 differentiation of IFN-γ+ cells during the effector phase of the response, we measured expression of Th1 markers on CD4 T cells at timepoints leading up to the peak of P. chabaudi infection

Summary

Introduction

Immunity against intracellular pathogens, such as the blood-stages of the rodent malaria parasite Plasmodium chabaudi, requires both antibodies and Th1-type responses [1, 2]. The CD4 T cell response starts with strong IFN-γ production, which reduces the initial parasite growth, followed by a marked change in the response to promote antibody and B cell involvement This is critical because antibody is required for complete parasite clearance [4]. During the early phases of this infection there is a switch in the type of antigen presenting cells, which reduces IFN-γ production [6] This change in T cell function includes acquiring the ability to secrete the regulatory cytokine IL-10, and the antibody-promoting cytokine IL-21 [7, 8]. This response seems appropriate to achieve an adequate balance between parasite control and immunopathology. Understanding the generation of IFN-γ-producing memory T cells is important for the rational creation of a malaria vaccine

Methods

Results

Conclusion