Viral pathogens often generate strong CD4+ T cell responses that are best known for their ability to help B cell and CD8+ T cell responses. However, recent studies reveal additional roles for CD4+ T cells, some of which are independent of other lymphocytes, and indicate that memory cells are more effective in most functions than naïve CD4 T cells. Here, we review the full spectrum of antiviral functions of CD4+ T cells, including their helper activities, innate immune induction, and direct anti-viral roles, and suggest how these functions are integrated to provide highly protective responses against viral pathogens.

We show here that IL-17-secreting CD4 T (Th)17 and CD8 T (Tc)17 effector cells are found in the lung following primary challenge with influenza A and that blocking Ab to IL-17 increases weight loss and reduces survival. Tc17 effectors can be generated in vitro using naive CD8 T cells from OT-I TCR-transgenic mice. T cell numbers expand 20-fold and a majority secretes IL-17, but little IFN-γ. Many of the IL-17-secreting cells also secrete TNF and some secrete IL-2. Tc17 are negative for granzyme B, perforin message, and cytolytic activity, in contrast to Tc1 effectors. Tc17 populations express message for orphan nuclear receptor γt and FoxP3, but are negative for T-bet and GATA-3 transcription factors. The FoxP3-positive, IL-17-secreting and IFN-γ-secreting cells represent three separate populations. The IFN-γ-, granzyme B-, FoxP3-positive cells and cells positive for IL-22 come mainly from memory cells and decrease in number when generated from CD44low rather than unselected CD8 T cells. Cells of this unique subset of CD8 effector T cells expand greatly after transfer to naive recipients following challenge and can protect them against lethal influenza infection. Tc17 protection is accompanied by greater neutrophil influx into the lung than in Tc1-injected mice, and the protection afforded by Tc17 effectors is less perforin but more IFN-γ dependent, implying that different mechanisms are involved.

We examined the expression and influence of IL-10 during influenza infection. We found that IL-10 does not impact sublethal infection, heterosubtypic immunity, or the maintenance of long-lived influenza Ag depots. However, IL-10-deficient mice display dramatically increased survival compared with wild-type mice when challenged with lethal doses of virus, correlating with increased expression of several Th17-associated cytokines in the lungs of IL-10-deficient mice during the peak of infection, but not with unchecked inflammation or with increased cellular responses. Foxp3− CD4 T cell effectors at the site of infection represent the most abundant source of IL-10 in wild-type mice during high-dose influenza infection, and the majority of these cells coproduce IFN-γ. Finally, compared with predominant Th1 responses in wild-type mice, virus-specific T cell responses in the absence of IL-10 display a strong Th17 component in addition to a strong Th1 response and we show that Th17-polarized CD4 T cell effectors can protect naive mice against an otherwise lethal influenza challenge and utilize unique mechanisms to do so. Our results show that IL-10 expression inhibits development of Th17 responses during influenza infection and that this is correlated with compromised protection during high-dose primary, but not secondary, challenge.

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Inflammation induced by recognition of pathogen-associated molecular patterns dramatically impacts subsequent adaptive responses. We asked if the adaptive immune system can also affect the character and magnitude of innate inflammatory responses. We find that the response of memory, but not naïve, CD4+ T cells enhances production of multiple innate inflammatory cytokines and chemokines (IIC) in the lung, and that during influenza infection, this leads to early control of virus. Memory CD4+ T cell induced IIC and viral control require cognate antigen recognition and are optimal when memory cells are either T helper type 1 (TH1)- or TH17-polarized, but are independent of interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) production and do not require activation of conserved pathogen recognition pathways. This represents a novel mechanism by which memory CD4+ T cells induce an early innate response that enhances immune protection against pathogens.

It is unclear how CD4 T cell memory formation is regulated following pathogen challenge, and when critical mechanisms act to determine effector T cell fate. Here, we report that following influenza infection most effectors require signals from major histocompatibility complex class II molecules and CD70 during a late window well after initial priming to become memory. During this timeframe, effector cells must produce IL-2 or be exposed to high levels of paracrine or exogenously added IL-2 to survive an otherwise rapid default contraction phase. Late IL-2 promotes survival through acute down regulation of apoptotic pathways in effector T cells and by permanently upregulating their IL-7 receptor expression, enabling IL-7 to sustain them as memory T cells. This new paradigm defines a late checkpoint during the effector phase at which cognate interactions direct CD4 T cell memory generation.

SummaryWe have outlined the carefully orchestrated process of CD4 + T-cell differentiation from naïve to effector and from effector to memory cells with a focus on how these processes can be studied in vivo in responses to pathogen infection. We emphasize that the regulatory factors that determine the quality and quantity of the effector and memory cells generated include (i) the antigen dose during the initial T-cell interaction with antigen-presenting cells; (ii) the dose and duration of repeated interactions; and (iii) the milieu of inflammatory and growth cytokines that responding CD4 + T cells encounter. We suggest that heterogeneity in these regulatory factors leads to the generation of a spectrum of effectors with different functional attributes. Furthermore, we suggest that it is the presence of effectors at different stages along a pathway of progressive linear differentiation that leads to a related spectrum of memory cells. Our studies particularly highlight the multi-faceted roles of CD4 + effector and memory T cells in protective responses to influenza infection and support the concept that efficient priming of CD4 + T cells that react to shared influenza proteins could contribute greatly to vaccine strategies for influenza. Overview and historyOver the past decade, others and we have concluded that naïve precursor T cells must undergo many steps of division and differentiation before they acquire the effector functions necessary for their many regulatory activities (1). One of these activities is 'help' for B cells, which promotes B-cell isotype switching, somatic mutation, and differentiation in germinal centers to plasma cells and memory cells (2-4). Another key regulatory activity carried out by CD4 + T cells involves help for naïve CD8 + T cells to promote their optimum differentiation into cytotoxic effectors and memory cells and to support their maintenance (5-7). In addition, there are a host of other regulatory effects of CD4 + effectors on macrophages as well as other antigenpresenting cells (APCs). These CD4 + T-cell functions are mediated by surface coreceptors on the effector cells, including CD40L, CD28, cytotoxic T-lymphocyte antigen-4, etc., that interact with receptors on B cells, dendritic cells, macrophages, or other APCs, and by potent cytokines secreted by the CD4 + effectors upon recognition of antigen on APCs.CD4 + T-cell effectors represent a collection of distinct subsets characterized in part by their abilities to produce different patterns of cytokines. The two best characterized subsets are designated T-helper 1 (Th1), producing interferon-γ (IFN-γ), and Th2, producing interleukin-4 (IL-4), IL-5, and IL-13 as 'signature' cytokines. Recently, evidence has accumulated for a third . Most probably the APCs that stimulate the naïve CD4 + T cells are also the initial source of cytokines that imprint these subsets in situ (11). It is also increasingly accepted that the polarizing cytokines secreted by the APCs are dictated by the context of the antigen, be it from a pathogen or...

We show in this study several novel features of T cell-based heterosubtypic immunity against the influenza A virus in mice. First, T cell-mediated heterosubtypic protection against lethal challenge can be generated by a very low priming dose. Second, it becomes effective within 5–6 days. Third, it provides protection against a very high dose challenge for >70 days. Also novel is the finding that strong, long-lasting, heterosubtypic protection can be elicited by priming with attenuated cold-adapted strains. We demonstrate that priming does not prevent infection of the lungs following challenge, but leads to earlier clearance of the virus and 100% survival after otherwise lethal challenge. Protection is dependent on CD8 T cells, and we show that CD4 and CD8 T cells reactive to conserved epitopes of the core proteins of the challenge virus are present after priming. Our results suggest that intranasal vaccination with cold-adapted, attenuated live virus has the potential to provide effective emergency protection against emerging influenza strains for several months.