Nonspecific CD8 ⁺ T Cells and Dendritic Cells/Macrophages Participate in Formation of CD8 ⁺ T Cell-Mediated Clusters against Malaria Liver-Stage Infection

Abstract

CD8 T cells are the major effector cells that protect against malaria liver-stage infection, forming clusters around -infected hepatocytes and eliminating parasites after a prolonged interaction with these hepatocytes. We aimed to investigate the roles of specific and nonspecific CD8 T cells in cluster formation and protective immunity. To this end, we used ANKA expressing ovalbumin as well as CD8 T cells from transgenic mice expressing a T cell receptor specific for ovalbumin (OT-I) and CD8 T cells specific for an unrelated antigen, respectively. While antigen-specific CD8 T cells were essential for cluster formation, both antigen-specific and nonspecific CD8 T cells joined the clusters. However, nonspecific CD8 T cells did not significantly contribute to protective immunity. In the livers of infected mice, specific CD8 T cells expressed high levels of CD25, compatible with a local, activated effector phenotype. imaging of the liver revealed that specific CD8 T cells interact with CD11c cells around infected hepatocytes. The depletion of CD11c cells virtually eliminated the clusters in the liver, leading to a significant decrease in protection. These experiments reveal an essential role of hepatic CD11c dendritic cells and presumably macrophages in the formation of CD8 T cell clusters around -infected hepatocytes. Once cluster formation is triggered by parasite-specific CD8 T cells, specific and unrelated activated CD8 T cells join the clusters in a chemokine- and dendritic cell-dependent manner. Nonspecific CD8 T cells seem to play a limited role in protective immunity against parasites.

Clustering of activated CD8 T cells around malaria-infected hepatocytes is rapid and is driven by antigen-specific T cells

“…Mathematical modeling also suggested that there may be a slight bias in the clusters to have more Py-specific T cells than T cells of irrelevant specificity per cluster but the biological relevance of such a small bias remains unclear. The limited role of T cells of irrelevant specificity in the formation of T cell clusters in Py-infected mice is consistent with the observation that transfers of large numbers of activated CD8 T cells with irrelevant specificity into Plasmodium-infected mice did not impact efficiency at which parasites were killed by Plasmodium-specific T cells [31]. Interestingly, and perhaps surprisingly, this result contradicts a recent observation of suppression of development of T-cell-driven type 1 diabetes by islet-non specific CD8 T cells [50].…”

“…Our results suggest that activated CD8 T cells of irrelevant specificities do not play a major role in cluster formation, and elegant experiments demonstrated that large numbers of non-specific T cells do not impair the ability of Plasmodium-specific T cells to eliminate the parasites [31]. However, the latter result was found by using only two different ratios of Plasmodium-specific and non-specific T cells and 3 mice per group, so it remains to be determined if competition between such cells for the access to infected cells occurs at higher ratios, e.g., as has been observed in another system [50].…”

“…These results suggested that data on clustering of Py-specific T cells alone may be insufficient to discriminate between T cell-intrinsic and T cell-extrinsic mechanisms of cluster formation [49]. A key experiment, rejecting the “variability of the environment” hypothesis as the sufficient mechanism explaining distribution of cluster sizes is one involving either transfer of only OT1 T cells (which are not specific to Py antigens) or OT1 T cells together with PyTCR T cells – only in the latter case, OT1 T cells form co-clusters with Py-specific T cells (Figure 4 and see [29, 31]). The mathematical model assuming fixed yet variable (between individual parasites) environment was not able to accurately explain such data (Figure 4).…”

“…Recent studies utilizing fluorescently labeled sporozoites and activated Plasmodium-specific CD8 T cells and intravital microscopy revealed clustering of CD8 T cells near the parasite in the mouse livers whereby multiple T cells were located in close proximity (≤ 40 μ m) of some liver stages [23, 29–31]. Interestingly, we observed that clustering of T cells near the parasite results in a higher chances of parasite’s death suggesting that clusters may increase the efficiency at which T cells eliminate the infection.…”

Immune effector mechanisms in malaria: An update focusing on human immunity

“…120 In the context of P. vivax, contact-dependent CD8+ T cell cytotoxicity of parasitized-reticulocytes is mediated by granulysins. 121 There is also evidence of protective CD8+ T cell responses against sporozoites, 122,123 although protection induced by the RTS,S vaccine (expressing CSP) is mediated by CD4+ T cells in conjunction with antibody responses. 124 However, RBCs lack MHC molecules and pathways necessary to process and present antigens directly to T cells.…”