CD4+ T cells license Kupffer cells to reverse the CD8+ T cell dysfunction induced by hepatocellular priming

CD4? T cells are believed to be essential for effective CD8? T cell responses against hepatotropic viruses like hepatitis B virus (HBV). However, the specific locations and the cellular and molecular mechanisms through which CD4? T cells help CD8? T cells remain unclear. In this study, we generated HBV-specific CD4? TCR transgenic mice to demonstrate that effector CD4? T cells can prevent or reverse the dysfunction of CD8? T cells caused by hepatocellular priming. This, in turn, enhances CD8? T cell effector function and suppresses viral replication. Notably, CD4? T cell help to CD8+ T cells occurs directly in the liver, independent of secondary lymphoid organs, and requires local antigen recognition but not epitope linkage. While dendritic cells were found to be dispensable, Kupffer cells (KCs) emerged as the critical cellular platform for this effect. Mechanistically, CD4? T cells engage KCs via CD40-CD40L interactions, leading to the production of IL-12 and IL-27. IL-12 promotes further CD4? T cell expansion, whereas IL-27 serves as the key cytokine that restores CD8? T cell functionality. Furthermore, we show that exogenous IL-27 administration can reverse HBV-specific CD8? T cell dysfunction in mice and in chronically infected patients. These findings reveal a novel mechanism of CD4? T cell help to CD8? T cells that occurs outside secondary lymphoid organs, specifically in the liver, and involves antigen-presenting cells other than conventional dendritic cells, offering new insights for immunotherapeutic strategies against chronic HBV infection. Overall design: We generated an HBV-specific CD4+ TCR transgenic (Tg) mouse model. Wild-type (WT) mice were immunized with the I-Ab-restricted immunodominant Env126-138 peptide from the preS2 region of the HBV genome. After immunization, we isolated splenocytes and restimulated them ex vivo with the Env126-138 peptide. Single-cell sorting and TCR sequencing of IFN-? producing CD4+ T cells identified two distinct variable TCR region genes for the alpha and beta chains. These genes were cloned into TCR expression vectors known for efficient TCR gene expression. The vectors were co-injected into fertilized C57BL/6 eggs, generating two transgenic mouse lines expressing the alpha and beta chains of the Env126-138 TCR. These lines were crossbred to produce HBV-specific CD4+ TCR transgenic mice (referred to as Env126 mice).