Although aimed at controlling invading pathogens, immune responses can sometimes be harmful to the host. Responses against the blood stage of malaria are an example of this: while B cells are activated and produce useful antibodies that kill parasites, killer CD8⁺T cells are also generated that inadvertently cause damage to the brain (a pathology called cerebral malaria), which may be lethal. CD4⁺T cells (or “helper” T cells) are key orchestrators of these immune responses, helping B cells make antibodies and activating another type of immune cell called a dendritic cell, which in turn stimulates killer T cell responses. Understanding the mechanisms by which CD4⁺T cells control immunity is therefore important for the design of strategies to enhance immunity and prevent malaria pathology.

An important difficulty when studying T cells is the fact that we (and mice) have tens of millions of them, with only a few (sometimes less than a hundred) recognising the same target (antigen). Thus, it is very hard for us to study the rare T cells specific for malaria during infection. T cell receptor-transgenic mice provide a solution for this, as all T cells in these mice are forced to express the same T cell receptor (TCR; the molecule that determines the antigen specificity of a T cell), so they all recognise the same antigen (for example, a malaria antigen). These cells usually carry a marker that enables us to distinguish them from the millions of normal T cells (mostly non-antigen specific) after we transfer them into a normal mouse. We made a CD4⁺T cell TCR-transgenic mouse line specific for malaria that we termed PbT-II. This mouse and its unique population of malaria-specific T cells enabled us to study in great detail the function of CD4⁺T cells during the course of malaria.

We first characterised our PbT-II cells and showed that they are reactive to multiple species of malaria parasites, even recognisingPlasmodium falciparumparasites that normally infect humans. They also respond to malaria parasites that grow the liver, prior to blood stage infection. This means, PbT-II cells are a very versatile tool to study responses against malaria in multiple contexts.

For CD4⁺T cells to work as helpers, they need to get activated themselves. We sought to determine which cells were responsible for the activation of PbT-II cells during blood stage malaria – which we measured by quantifying PbT-II division in the presence or absence of certain antigen presenting cells. We established that dendritic cells (DC), and more specifically a subset called CD8⁺DC, were most efficient at inducing PbT-II cell proliferation. Moreover, CD8⁺DC promoted the differentiation of activated PbT-II cells into Th1 cells (a CD4⁺T cell subset that induces inflammation) and Tfh cells (follicular helper T cells; another subset specialised in stimulating antibody production).

Once activated, CD4⁺T cells interact with B cells and dendritic cells, enabling them to produce effective antibodies and to activate killer T cells against the invading pathogen. By using gene deficient mice, we demonstrated that PbT-II cells utilise the surface molecule CD40L, which binds its receptor CD40 on B cells and dendritic cells, to carry out this function. While B cells activated in this way produced antibodies that eliminated the parasites from the blood, dendritic cells stimulated killer T cells that accelerated the onset of cerebral malaria.

In conclusion, we generated an important tool for the study of immune responses against malaria and used it to better understand CD4⁺T cell activation and the subsequent, dual role of activated CD4⁺在bloo T细胞免疫和病理d stage malaria.

Daniel Fernandez Ruiz, William R Heath
Department of Microbiology and Immunology, Peter Doherty Institute for Infection and Immunity,
University of Melbourne, Melbourne, Australia

Publication

Development of a Novel CD4+ TCR Transgenic Line That Reveals a Dominant Role for CD8+ Dendritic Cells and CD40 Signaling in the Generation of Helper and CTL Responses to Blood-Stage Malaria.
Fernandez-Ruiz D, Lau LS1, Ghazanfari N, Jones CM, Ng WY, Davey GM, Berthold D, Holz L, Kato Y, Enders MH, Bayarsaikhan G, Hendriks SH, Lansink LIM, Engel JA, Soon MSF, James KR, Cozijnsen A, Mollard V, Uboldi AD, Tonkin CJ, de Koning-Ward TF, Gilson PR, Kaisho T, Haque A, Crabb BS, Carbone FR, McFadden GI, Heath WR
J Immunol. 2017 Dec 15

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