Ex Pluribus Unum: The CD4 T Cell Response against Influenza A Virus

Current Influenza A virus (IAV) vaccines, which primarily aim to generate neutralizing antibodies against the major surface proteins of specific IAV strains predicted to circulate during the annual 'flu' season, are suboptimal and are characterized by relatively low annual vaccine efficacy. One approach to improve protection is for vaccines to also target the priming of virus-specific T cells that can protect against IAV even in the absence of preexisting neutralizing antibodies. CD4 T cells represent a particularly attractive target as they help to promote responses by other innate and adaptive lymphocyte populations and can also directly mediate potent effector functions. Studies in murine models of IAV infection have been instrumental in moving this goal forward. Here, we will review these findings, focusing on distinct subsets of CD4 T cell effectors that have been shown to impact outcomes. This body of work suggests that a major challenge for next-generation vaccines will be to prime a CD4 T cell population with the same spectrum of functional diversity generated by IAV infection. This goal is encapsulated well by the motto 'ex pluribus unum': that an optimal CD4 T cell response comprises many individual specialized subsets responding together.

STAT1 Controls the Functionality of Influenza-Primed CD4 T Cells but Therapeutic STAT4 Engagement Maximizes Their Antiviral Impact

It is generally accepted that influenza A virus (IAV) infection promotes a Th1-like CD4 T cell response and that this effector program underlies its protective impact. Canonical Th1 polarization requires cytokine-mediated activation of the transcription factors STAT1 and STAT4 that synergize to maximize the induction of the "master regulator" Th1 transcription factor, T-bet. Here, we determine the individual requirements for these transcription factors in directing the Th1 imprint primed by influenza infection in mice by tracking virus-specific wild-type or T-bet-deficient CD4 T cells in which STAT1 or STAT4 is knocked out. We find that STAT1 is required to protect influenza-primed CD4 T cells from NK cell-mediated deletion and for their expression of hallmark Th1 attributes. STAT1 is also required to prevent type I IFN signals from inhibiting the induction of the Th17 master regulator, Rorγt, in Th17-prone T-bet-/- cells responding to IAV. In contrast, STAT4 expression does not appreciably impact the phenotypic or functional attributes of wild-type or T-bet-/- CD4 T cell responses. However, cytokine-mediated STAT4 activation in virus-specific CD4 T cells enhances their Th1 identity in a T-bet-dependent manner, indicating that influenza infection does not promote maximal Th1 induction. Finally, we show that the T-bet-dependent protective capacity of CD4 T cell effectors against IAV is optimized by engaging both STAT1 and STAT4 during Th1 priming, with important implications for vaccine strategies aiming to generate T cell immunity.

Macromolecules Absorbed from Influenza Infection-Based Sera Modulate the Cellular Uptake of Polymeric Nanoparticles

Optimizing the biological identity of nanoparticles (NPs) for efficient tumor uptake remains challenging. The controlled formation of a protein corona on NPs through protein absorption from biofluids could favor a biological identity that enables tumor accumulation. To increase the diversity of proteins absorbed by NPs, sera derived from Influenza A virus (IAV)-infected mice were used to pre-coat NPs formed using a hyperbranched polyester polymer (HBPE-NPs). HBPE-NPs, encapsulating a tracking dye or cancer drug, were treated with sera from days 3-6 of IAV infection (VS3-6), and uptake of HBPE-NPs by breast cancer cells was examined. Cancer cells demonstrated better uptake of HBPE-NPs pre-treated with VS3-6 over polyethylene glycol (PEG)-HBPE-NPs, a standard NP surface modification. The uptake of VS5 pre-treated HBPE-NPs by monocytic cells (THP-1) was decreased over PEG-HBPE-NPs. VS5-treated HBPE-NPs delivered a cancer drug more efficiently and displayed better in vivo distribution over controls, remaining stable even after interacting with endothelial cells. Using a proteomics approach, proteins absorbed from sera-treated HBPE-NPs were identified, such as thrombospondin-1 (TSP-1), that could bind multiple cancer cell receptors. Our findings indicate that serum collected during an immune response to infection is a rich source of macromolecules that are absorbed by NPs and modulate their biological identity, achieving rationally designed uptake by targeted cell types.

Lung-resident memory CD4 T cells recognizing antigen in the airway enhance the kinetics of antigen presentation and naive T cell activation in draining lymph nodes

CD4+ tissue-resident memory (TRM) T cells act as sentinels at previous sites of infection and rapidly initiate protective inflammatory responses upon antigen re-encounter. Here, we tested if beyond local impacts in the lung, TRM responses lead to faster egress of antigen-loaded dendritic cells (DC) resulting in their more rapid accumulation in draining lymph nodes (DLN). To do so, we used a mouse model originally developed to examine CD4+ TRM-mediated protection against influenza virus infection. We established a TRM population in otherwise naïve mice by intranasal transfer of memory CD4 T cells specific for ovalbumin (OVA). The mice were then challenged with fluorescently-labelled OVA protein intranasally. Faster migration from the lung to the DLN by OVA+ DC was seen within 48 hours of protein administration in mice receiving TRM compared to controls without TRM transfer. We hypothesized that the enhanced kinetics of OVA presentation in DLN mediated by TRM antigen recognition would result in more rapid systemic activation of OVA-specific CD4 T cells. We thus transferred an indicator population of naïve OVA-specific CD4 T cells intravenously to mice receiving TRM transfer or not prior to OVA protein administration. Faster kinetics of naïve T cell activation were seen in recipients of TRM within 3 days as judged by expression of activation markers and proliferation. Most importantly, by day 4, responding naïve cells reached the lung in greater numbers and were more functional in mice receiving TRM. Our results reveal that CD4+ TRM activation can enhance the kinetics of antigen presentation in DLNs and suggest this mechanism can be harnessed by vaccines to fully integrate protective local and systemic CD4 T cell responses.

Supported by grants from NIH (R21 R21AI148882)

Bona Fide Th17 Cells without Th1 Functional Plasticity Protect against Influenza

Optimal transcriptional programming needed for CD4 T cells to protect against influenza A virus (IAV) is unclear. Most IAV-primed CD4 T cells fit Th1 criteria. However, cells deficient for the Th1 "master regulator," T-bet, although marked by reduced Th1 identity, retain robust protective capacity. In this study, we show that T-bet's paralog, Eomesodermin (Eomes), is largely redundant in the presence of T-bet but is essential for the residual Th1 attributes of T-bet-deficient cells. Cells lacking both T-bet and Eomes instead develop concurrent Th17 and Th2 responses driven by specific inflammatory signals in the infected lung. Furthermore, the transfer of T-bet- and Eomes-deficient Th17, but not Th2, effector cells protects mice from lethal IAV infection. Importantly, these polyfunctional Th17 effectors do not display functional plasticity in vivo promoting gain of Th1 attributes seen in wild-type Th17 cells, which has clouded evaluation of the protective nature of Th17 programming in many studies. Finally, we show that primary and heterosubtypic IAV challenge is efficiently cleared in T-bet- and Eomes double-deficient mice without enhanced morbidity despite a strongly Th17-biased inflammatory response. Our studies thus demonstrate unexpectedly potent antiviral capacity of unadulterated Th17 responses against IAV, with important implications for vaccine design.

Highly protective T-h17-polarized responses against Influenza A Virus develop in the absence of CD4 T cell-intrinsic T-bet and Eomes expression

Though bulk CD4 T cell responses against influenza A virus (IAV) are characterized as Th1, expression of the transcription factor T-bet (Tbx21), the ‘master regulator’ of Th1 fate, is not required for mice to clear IAV. While the Tbx21−/− CD4 T cells develop some Th17 characteristics in the infected lung, they still retain relatively high Th1 functionality including robust IFNγ production. Here, we show that the transcription factor Eomesodermin (Eomes) is required for the residual Th1 identity of Tbx21−/− cells by comparing Tbx21−/− and double Tbx21−/−Eomes− /− CD4 T cells in an adoptive transfer model of IAV infection. The Tbx21−/−Eomes−/− cells not only lose Th1 attributes but develop much stronger Th17 attributes versus Tbx21−/− cells. The elaboration of Th17 responses by Tbx21−/− Eomes−/− cells is not a default effector state but requires receipt of both IL-6 and TGFβ in IAV-infected lungs. Similar Th17 identity in the absence of Th1 attributes are seen analyzing CD4 T cells in Tbx21−/−Eomes−/− mice infected with IAV, and this response correlates with protection as CD4 T cell depletion abrogates viral control. To more stringently test the protective capacity of Th17 responses, we transferred in-vitro Th17 primed WT or Tbx21−/−Eomes−/− effectors to unprimed WT hosts and challenged with lethal IAV. Both populations protected, but while WT Th17 cells gained Th1 characteristics in vivo, Tbx21−/−Eomes−/− cells strengthened their Th17 functionality remarkably without any shift towards Th1 identity. Our observations thus show that prototypical Th17 cells, that cannot gain Th1 attributes through mechanisms of plasticity often seen during pathogen responses, can mediate strong protection against IAV infection.

STAT1 protects CD4 T cells from deletion by NK cells during influenza infection and promotes Th1 identity that can be enhanced through STAT4 to improve their anti-viral impact

Most CD4 T cells responding to influenza A virus (IAV) fit prototypical Th1 criteria including high expression of the transcription factor T-bet, the Th1 ‘master regulator’, and strong IFNg production. Optimal Th1 development in vitro also requires cytokine-dependent activation of the transcription factors STAT4 and STAT1, but how these pathways impact anti-viral responses is not clear. Here, to focus on the roles of CD4 T cell-intrinsic STAT4 and STAT1, we transferred STAT-deficient (STAT−/−) or wildtype CD4 T cells recognizing virus to congenic wildtype hosts, challenged with IAV, and analyzed donor effector responses in the infected lungs. STAT4−/− and wildtype cells reached similar numbers with no differences seen in their Th1 identity. In contrast, STAT1−/− donor cells were barely detected in host mice but reached similar numbers to wildtype donors when NK cells were depleted. Analysis of STAT1−/− cells in NK cell-depleted hosts revealed severely compromised Th1 identity that mirrored T-bet−/− donor cell responses. While these findings suggest that full Th1-polarization during IAV infection does not require STAT4, treatment of mice with IL-12 dramatically increases Th1 attributes in wildtype but not STAT4−/−CD4 T cells. This indicates that the wildtype IAV-primed CD4 T cell response is not strongly Th1-polarized. Finally, priming CD4 T cells in vitro with STAT4- and STAT1-activating cytokines promotes effector cells better able to combat IAV when transferred to unprimed mice than when only STAT1 activating signals are present. Enhancing Th1 identity through harnessing CD4 T cell-intrinsic STAT4 activation can thus improve their anti-viral impact during infection, with important implications for vaccine design.

Lung-resident memory CD4 T cells do not divide extensively during protective responses against Influenza virus

Tissue-resident memory (TRM) CD4 T cells are emerging as critical orchestrators of immunity but little is known about their response kinetics during secondary challenges. We recently developed an adoptive transfer model in which virus-specific CD4 TRM isolated from lungs of Influenza A virus (IAV)-primed mice are transferred intranasally to new congenic hosts. Here, we track these donor TRM cells after IAV challenge of the adoptive hosts to gain insight into unique aspects of CD4 TRM responses to infection. Our findings demonstrate that the TRM rapidly activate elements of innate defense in the lung, including dendritic cells and NK cells. The TRM response is limited to the lung as no donor cells are detected in the spleen or mediastinal draining lymph nodes through the first week of infection. Although the CD4 TRM are highly protective and can clear even lethal IAV challenge in unprimed hosts, they undergo only limited division. To ask if the TRM-mediated protection requires influx of systemic T cells to the lung, we treated adoptive hosts with FTY720 to block the migration of host T cells during infection. TRM-mediated protection was not impacted by FTY720 treatment, indicating that the local memory CD4 cells can protect independently of T cell responses initiated in secondary lymphoid organs. Finally, we could detect the donor TRM in lungs for at least 21 days after infection and they were rapidly activated by antigen given intranasally. Our results thus show that lung CD4 TRM initiate highly protective responses in the lungs of otherwise naïve mice and have the capacity to respond against multiple rounds of infection. As such their generation through vaccination is a powerful way to improve immunity against seasonal and pandemic IAV.

Cigarette smoke extract acts directly on CD4 T cells to enhance Th1 polarization and reduce memory potential

Although cigarette smoke is known to alter immune responses, whether and how CD4 T cells are affected is not well-described. We aimed to characterize how exposure to cigarette smoke extract impacts CD4 T cell effector generation in vitro under Th1-polarizing conditions. Our results demonstrate that cigarette smoke directly acts on CD4 T cells to impair effector expansion by decreasing division and increasing apoptosis. Furthermore, cigarette smoke enhances Th1-associated cytokine production and increases expression of the transcription factor T-bet, the master regulator of Th1 differentiation. Finally, we show that exposure to cigarette smoke extract during priming impairs the ability of effectors to form memory cells. Our findings thus demonstrate that cigarette smoke simultaneously enhances effector functions but promotes terminal differentiation of CD4 T cell effectors. This study may be relevant to understanding how smoking can both aggravate autoimmune symptoms and reduce vaccine efficacy.

Direct IL-6 Signals Maximize Protective Secondary CD4 T Cell Responses against Influenza

Memory T cells can often respond against pathogens that have evaded neutralizing Abs and are thus key to vaccine-induced protection, yet the signals needed to optimize their responses are unclear. In this study, we identify a dramatic and selective requirement for IL-6 to achieve optimal memory CD4 T cell recall following heterosubtypic influenza A virus (IAV) challenge of mice primed previously with wild-type or attenuated IAV strains. Through analysis of endogenous T cell responses and adoptive transfer of IAV-specific memory T cell populations, we find that without IL-6, CD4+, but not CD8+, secondary effector populations expand less and have blunted function and antiviral impact. Early and direct IL-6 signals to memory CD4 T cells are required to program maximal secondary effector responses at the site of infection during heterosubtypic challenge, indicating a novel role for a costimulatory cytokine in recall responses.

The metabolism of salicylidene benzylamine, a pro-drug for salicylate, in rats and dogs

1. Oral doses of [14C]salicylidene benzylamine were well absorbed by rats and dogs and excreted mainly in the urine (92% dose by rat in 24 h and 84% by dog in 48 h). 2. After 50, 100 and 300 mg/kg oral doses to rats, peak mean plasma 14C concn. (mainly salicylic acid) of 77, 243 and 396 micrograms equiv./ml respectively were reached between 2 and 4 h and maintained at this level until about 7.5 h. 14C was cleared more rapidly from the plasma of male rats than from that of female rats. After 44, 85 and 300 mg/kg oral doses to dogs, peak mean plasma 14C concn. (mainly salicylic acid) of 114, 194 and 320 micrograms equiv./ml respectively were reached at 4 h and then declined with a half-life of 6-7 h. 3. Tissue 14C concn. in both species were greatest at the times of peak plasma 14C concn., but in general were lower than those in the corresponding plasma. 4. The major components in rat and dog urine were salicyl alcohol, salicylic acid and salicyluric acid, the first two being excreted partly as glucuronic acid/sulphate conjugates. After initial hydrolysis of salicylidene benzylamine to salicylaldehyde, approximately equal amounts were reduced and oxidized to the corresponding alcohol and acid respectively.