Cytotoxic CD4 development requires CD4 effectors to concurrently recognize local antigen and encounter type I IFN-induced IL-15

Cytotoxic CD4 T cell effectors (ThCTLs) kill virus-infected major histocompatibility complex (MHC) class II+ cells, contributing to viral clearance. We identify key factors by which influenza A virus infection drives non-cytotoxic CD4 effectors to differentiate into lung tissue-resident ThCTL effectors. We find that CD4 effectors must again recognize cognate antigen on antigen-presenting cells (APCs) within the lungs. Both dendritic cells and B cells are sufficient as APCs, but CD28 co-stimulation is not needed. Optimal generation of ThCTLs requires signals induced by the ongoing infection independent of antigen presentation. Infection-elicited type I interferon (IFN) induces interleukin-15 (IL-15), which, in turn, supports CD4 effector differentiation into ThCTLs. We suggest that these multiple spatial, temporal, and cellular requirements prevent excessive lung ThCTL responses when virus is already cleared but ensure their development when infection persists. This supports a model where continuing infection drives the development of multiple, more differentiated subsets of CD4 effectors by distinct pathways.

IgD+ age-associated B cells are the progenitors of the main T-independent B cell response to infection that generates protective Ab and can be induced by an inactivated vaccine in the aged

Age-associated B cells (ABC) accumulate with age and are associated with autoimmunity and chronic infection. However, their contributions to acute infection in the aged and their developmental pathways are unclear. We find that the response against influenza A virus infection in aged mice is dominated by a Fas+ GL7- effector B cell population we call infection-induced ABC (iABC). Most iABC express IgM and include antibody-secreting cells in the spleen, lung, and bone marrow. We find that in response to influenza, IgD+ CD21- CD23- ABC are the precursors of iABC and become memory B cells. These IgD+ ABC develop in germ-free mice, so are independent of foreign antigen recognition. The response of ABC to influenza infection, resulting in iABC, is T cell independent and requires both extrinsic TLR7 and TLR9 signals. In response to influenza infection, IgD+ ABC can induce a faster recovery of weight and higher total anti-influenza IgG and IgM titers that can neutralize virus. Immunization with whole inactivated virus also generates iABC in aged mice. Thus, in unimmunized aged mice, whose other B and T cell responses have waned, IgD+ ABC are likely the naive B cells with the potential to become Ab-secreting cells and to provide protection from infection in the aged.

CD4 Effectors must re-encounter Antigen Locally in the Presence of Signals from Infection to Fully Differentiate into Specialized Tissue-restricted TFH and ThCTL effectors

Functionally specialized, tissue-restricted CD4 effector subsets such as T helper cytotoxic cells (ThCTL) in the infected tissue and T follicular helper cells (TFH) in secondary lymphoid organs develop after Th1 and other effectors. Following influenza A virus infection, we find that CD4 effectors must recognize cognate Ag/MHC-II to become ThCTL and TFH but that this does not require a specific APC subset. While intrasplenic and intravenous Ag/APC delivery support spleen TFH, they do not support lung ThCTL or DLN TFH. Conversely, intranasal Ag/APC delivery supports lung ThCTL and DLN TFH indicating that Ag presentation in the site of residence is crucial. Strikingly, we find that continuing Ag-independent signals from infection are also needed for optimal generation of both subsets, even at this late effector phase. This contrasts with CD4 memory generation from effectors which is dependent only on Ag recognition and not on infection. Thus, this implies a pivotal checkpoint at which the fate of CD4 effectors is determined. In the absence of Ag most contract, while Ag alone can generate memory, and with continuing infection-mediated signals, distinct specialized effectors. This guards against unnecessary responses when pathogen is not present or has been cleared, but ensures the development of potent effectors in tissues if it persists. We propose that for vaccines to induce stronger immunity, they will need to be designed to provide additional presentation of CD4 T cell Ag and signals associated with infection at the effector checkpoint. This will provide the TFH needed for strong B cell Ab response and strong tissue-restricted CD4 T cell memory to ensure frontline response and broad, heterosubtypic protection.

Generation of tissue-restricted CD4 effectors (TFH and ThCTL) requires signals from local antigen & infection during the effector phase

Key specialized tissue-restricted CD4 effector subsets such as T helper cytotoxic cells (ThCTL) in the infected tissue and T follicular helper cells (TFH) in secondary lymphoid organs arise after other CD4 effectors (Th1, Th17) peak. In an influenza A virus (IAV) infection model, we find that generation of ThCTL and TFH require that CD4 effectors recognize specific Ag/MHC Class II, 5–7 days after initial infection. We restrict Ag presentation to specific APC subsets and add various APC via different routes, to study which APC and infection conditions drive TFH vs. ThCTL generation at the checkpoint. We find most MHC Class II-expressing APC subsets, including dendritic cells and B cells, are able to drive TFH and ThCTL generation. However, local Ag presentation is crucial. Strikingly, we find that continuing infection, even at this late effector phase, is needed for optimal generation of both subsets. This contrasts with CD4 memory generation, which in our previous studies, was not dependent on infection at this checkpoint. Thus, at the effector checkpoint, Ag presentation in the local tissue drives both optimal specialized effector and most memory generation, whilst signals from infection are required for optimum tissue-restricted effector differentiation. The infection signal can be provided by multiple PRR pathways. We suggest that vaccines will need to be designed to provide additional Ag presentation and the signals associated with infection at the effector checkpoint to induce optimum protective immune responses: to promote strong CD4 memory for broad, heterosubtypic protection and indirectly strong B cell response and memory leading to long-term humoral protection.

Generation of specialized CD4 subsets: TFH and ThCTL, requires signals from local antigen & infection during the effector phase

Certain key specialized tissue-restricted CD4 effector subsets such as cytotoxic T cells (ThCTL) in the infected tissue and follicular helper T cells (TFH) in secondary lymphoid organs reach their peak responses after other CD4 effectors (Th1, Th17). Using an influenza A virus (IAV) infection model, we find that generation of ThCTL and TFH require that CD4 effectors recognize specific Ag/MHC Class II, 5–7 days after initial infection, a phase we have termed the late Ag checkpoint. We used TcR transgenic CD4 donor T cells and host mice in which Ag presentation is restricted to specific APC, as well as addition of various APC subsets at the checkpoint, to study which APC drive TFH vs. ThCTL generation. While tissue-restricted antigen presentation is crucial, several APC subsets were able to present Ag effectively to drive TFH and ThCTL generation. However, we found that host infection during the late Ag checkpoint was needed for optimal generation of donor ThCTL and TFH subsets. This contrasts with most CD4 memory generation that is not dependent on infection. Thus multiple CD4 fate decisions are made at the late Ag checkpoint. Both optimal specialized effector and most memory generation require continuing Ag presentation, while further tissue-specific effector differentiation requires local Ag presentation and we think, signals from pathogen recognition pathways (PRR). We suggest that to induce optimum protective immune responses, vaccines and immunotherapies may need to be designed taking into account these requirements for additional Ag presentation and PRR pathways at the late Ag checkpoint.

NKG2C/E Marks the Unique Cytotoxic CD4 T Cell Subset, ThCTL, Generated by Influenza Infection

CD4 T cells can differentiate into multiple effector subsets, including ThCTL that mediate MHC class II-restricted cytotoxicity. Although CD4 T cell-mediated cytotoxicity has been reported in multiple viral infections, their characteristics and the factors regulating their generation are unclear, in part due to a lack of a signature marker. We show in this article that, in mice, NKG2C/E identifies the ThCTL that develop in the lung during influenza A virus infection. ThCTL express the NKG2X/CD94 complex, in particular the NKG2C/E isoforms. NKG2C/E⁺ ThCTL are part of the lung CD4 effector population, and they mediate influenza A virus-specific cytotoxic activity. The phenotype of NKG2C/E⁺ ThCTL indicates they are highly activated effectors expressing high levels of binding to P-selectin, T-bet, and Blimp-1, and that more of them secrete IFN-γ and readily degranulate than non-ThCTL. ThCTL also express more cytotoxicity-associated genes including perforin and granzymes, and fewer genes associated with recirculation and memory. They are found only at the site of infection and not in other peripheral sites. These data suggest ThCTL are marked by the expression of NKG2C/E and represent a unique CD4 effector population specialized for cytotoxicity.

Antigen Presenting Cells as Drivers of Specialized Immune Responses

We have defined the “Memory Checkpoint” as the period when CD4 effector T cells (TEFF) require cognate antigen (Ag) recognition for further differentiation into CD4 memory T cells. Using an influenza A virus (IAV) infection model, we find that generation of CD4 cytotoxic T cells (ThCTL) from TEFF requires MHC Class-II restricted Ag at this checkpoint, 5–7 days after initial infection. Ag recognition at this checkpoint also determines CD4 follicular helper T cell (TFH) generation in our model. We have used transgenic mouse models in which Ag presentation is restricted to specific antigen presenting cells (APC), bone marrow chimeras and addition of various APC subsets at the memory checkpoint to study which APC drive TEFF to TFH vs. ThCTL. Our studies indicate that distinct APC, during this checkpoint, promote different fates resulting in generation of distinct effector subsets. While, as expected, B cell APC are necessary for TFH responses, the APC subset(s) that promote ThCTL generation are not yet clear. Our data indicate that a specialized hematopoietic subset of CD11c+ APC, that are preferentially localized in the infected tissue, is required at this checkpoint, to support ThCTL generation. Once we identify the APC subset(s) needed, we plan to target Ag to particular APC at the checkpoint, to uniquely promote distinct functional subsets and evaluate their function. We suggest this may allow us to tailor the quality of the effector response to better protect against distinct pathogens.