The Threshold of Protection from Liver-Stage Malaria Relies on a Fine Balance between the Number of Infected Hepatocytes and Effector CD8+ T Cells Present in the Liver.

Alexandra J Spencer,Anita Gola,Teresa Lambe,Adrian V S Hill,Rhea J Longley,Marta Ulaszewska

doi:10.4049/jimmunol.1601209

Alexandra J Spencer, Anita Gola + Show 4 more

Open Access

https://doi.org/10.4049/jimmunol.1601209

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Abstract

Since the demonstration of sterile protection afforded by injection of irradiated sporozoites, CD8+ T cells have been shown to play a significant role in protection from liver-stage malaria. This is, however, dependent on the presence of an extremely high number of circulating effector cells, thought to be necessary to scan, locate, and kill infected hepatocytes in the short time that parasites are present in the liver. We used an adoptive transfer model to elucidate the kinetics of the effector CD8+ T cell response in the liver following Plasmodium berghei sporozoite challenge. Although effector CD8+ T cells require <24 h to find, locate, and kill infected hepatocytes, active migration of Ag-specific CD8+ T cells into the liver was not observed during the 2-d liver stage of infection, as divided cells were only detected from day 3 postchallenge. However, the percentage of donor cells recruited into division was shown to indicate the level of Ag presentation from infected hepatocytes. By titrating the number of transferred Ag-specific effector CD8+ T cells and sporozoites, we demonstrate that achieving protection toward liver-stage malaria is reliant on CD8+ T cells being able to locate infected hepatocytes, resulting in a protection threshold dependent on a fine balance between the number of infected hepatocytes and CD8+ T cells present in the liver. With such a fine balance determining protection, achieving a high number of CD8+ T cells will be critical to the success of a cell-mediated vaccine against liver-stage malaria.

Highlights

To induce a high number of Ag-specific (Pb9 or enhanced GFP (eGFP)) CD8+ T cells with a protective phenotype [22, 42], donor mice were vaccinated with a heterologous prime boost vaccination regimen comprising human adenovirus serotype 5 (HAd5) followed at least 6 wk later by a modified vaccinia Ankara (MVA) vaccination with vectors expressing either TIPeGFP or GFP, and cells were transferred 10 d after the MVA boost
Recipient mice were harvested on the day of challenge and days 1, 2 and 3 after challenge, and the responses in the liver and liver-draining lymph nodes [43, 44] and spleen were measured by intracellular cytokine staining (ICS)
A small proportion of Pb9-specific dividing cells was observed in the liver at day 2, but a distinct population of dividing cells was not observed in the liver until 3 d postchallenge (Fig. 1A), corresponding to a significant increase in the percentage of dividing IFN-g+ Pb9-specific cells in TIPeGFP recipient mice compared with IFN-g+ GFP-specific cells in GFP recipient controls (Fig. 1) or IFN-g+GFP+–specific cells in the same TIPeGFP recipients

Summary

Introduction

To determine the kinetics of the Ag-specific CD8+ T cell response to liver-stage malaria infection, splenocytes from viral vector vaccinated mice were CFSE labeled and transferred into recipient mice prior to i.v. challenge with 1000 sporozoites.

Results

Conclusion