iMATES: Immunotherapy’s Best Friend?

Building an effective immunotherapy means understanding the fine details of the immune response. Gaps in that understanding can keep promising therapies from ever reaching the clinic. In the classical model, cytotoxic T lymphocytes (CTLs) get activated by antigen presenting cells. These CTLs are primed to target cells that display a specific antigen on their Major Histocompatibility Complex I (MHC I)¹. They then expand and travel to the target site. This model would allow ex vivo expansion and activation of CTLs for therapeutic use.

But in adoptive cytotoxic T lymphocyte (CTL) therapy for virally-infected liver, signals in the infected organ’s microenvironment cause CTLs to become exhausted, inhibited, or to die. Clusters of macrophages called granulomas form after mycobacterial infection, and these are thought to activate CTL function². In a recent article in Nature Immunology, Huang et al. show that a unique subpopulation of inflammatory, monocytic myeloid-cell clusters can expand CTLs at infection sites without needing antigen. The researchers named these clusters intra-hepatic myeloid-cell aggregates for T cell population expansion (or iMATEs).

TLR Stimulation Drives CTL Proliferation in the Liver

The researchers first studied how inflammatory activation affects CTLs. They used ex-vivo activated, adoptively-transferred CD90.1⁺ CTLs in CD90.2⁺ mice. Then they treated the mice with ligands for Toll-like Receptor 9 (TLR9) and Toll-like Receptor 4 (TLR4). They measured:

• Changes in CTL proliferation
• Increases in myeloid (CD11b⁺, Major Histocompatibility Class II-positive (MHCII⁺)) populations

Higher numbers of CTLs appeared in the liver only when mice received TLR9 or TLR4 ligands. This shows that this stimulation drives CTL proliferation².

Huang et al. also noted that these proliferating cells were not spread evenly through the liver. When they tested mice with Tlr9 knocked out, the adoptively transferred CTLs did not proliferate at all. This led Huang et al. to propose that TLR9 activation must happen in cells other than CTLs. Because macrophages are sensitive to TLR9 stimulation, the researchers depleted the macrophage population and saw CTL proliferation drop in the liver². These monocytes also clustered together after TLR9 stimulation, matching a proliferative T-cell phenotype.

Identifying iMATEs and How They Form

To show that stimulated iMATEs help expand CTLs, Huang et al. tracked CTL proliferation using transferred CD90.1⁺ CTLs in CD90.2⁺ mice. Using intravital microscopy, immunohistochemical analysis, and electron microscopy, they measured changes in:

• CD8 (CTL marker)
• CD45.1 (B Cell Marker)
• CD11b (Innate Immune Cell marker)

The data showed that CTL proliferation was localized to the liver, and iMATE formation only needed TLR9 stimulation. TLR9-treated mice also had more resident monocyte cells (Kupffer cells) and more non-resident monocytic cells. Monocyte-derived Dendritic Cells (DCs) — not conventional DCs — appeared at the TLR9-driven inflammatory site. These cells were uniquely able to drive CTL proliferation without presenting antigen².

What Drives iMATE-Induced CTL Proliferation

Given the link between iMATE formation and CTL proliferation, the researchers asked what mechanism drives it. Examining the iMATE population for changes in costimulatory receptors, they found higher expression of CD80, CD86, and OX40L. OX40L did not appear in normal liver tissue, so the researchers suspected OX40L/OX40 signaling plays a key role in iMATE-induced CTL proliferation.

Huang et al. showed that TLR9-induced CTL proliferation only happened when CTLs expressed the OX40 receptor, and only at the liver site². They also found that OX40L may improve CTL survival, but it played no role in CTL proliferation itself. This suggests iMATEs’ main job may be to help incoming CTLs survive, through a pathway that does not need antigen.

Implications for Treating Chronic Viral Infection

Finally, the researchers tested whether the larger CTL populations from iMATE stimulation could improve control of viral infection in the liver. They used mice with an inflammatory response triggered by TLR9 ligand treatment. Across two virally infected mouse models and one human model, they found:

• Acute inflammation caused iMATE formation and increased CTL populations
• Chronic inflammation did not

Huang et al. also saw lower exhaustion markers — measured by PD1⁺ and CD69⁺ expression — in TLR9-stimulated mice. Combined with vaccination, this produced more activated, non-exhausted CTLs focused on the vaccination target². Finally, combining a vaccination regimen with TLR9 to form iMATEs cleared infected liver cells (hepatocytes) in a mouse model².

Together, these findings suggest that stimulating iMATEs may boost the body’s own CTL-driven anti-viral immune response. This could improve outcomes for immunotherapy treatment of chronic viral infection in the liver. It’s still unknown whether this happens at other virally-infected sites, or in other diseases¹. It’s also unclear whether this is unique to ex-vivo expanded and activated CTLs. CTLs activated naturally in the body may or may not go through the same change.

Still, the idea of changing a viral infection site’s inflammatory environment before adoptive cell therapy — to make it work better — is promising. TLR9-based therapy is already in clinical trials for other conditions. Combining these two immunotherapy techniques could lead to real advances in treating virally-infected diseases.

References:

1. Crispe, I. N. & Pierce, R. H. Killer T cells find meaningful encounters through iMATEs. Nature immunology 14, 533-534, doi:10.1038/ni.2620 (2013).

2. Huang, L. R. et al. Intrahepatic myeloid-cell aggregates enable local proliferation of CD8(+) T cells and successful immunotherapy against chronic viral liver infection. Nature immunology 14, 574-583, doi:10.1038/ni.2573 (2013).