Susan L. Swain

IL-23 and IL-17 in the establishment of protective pulmonary CD4+ T cell responses after vaccination and during Mycobacterium tuberculosis challenge.

Interferon-γ is key in limiting Mycobacterium tuberculosis infection. Here we show that vaccination triggered an accelerated interferon-γ response by CD4+ T cells in the lung during subsequent M. tuberculosis infection. Interleukin 23 (IL-23) was essential for the accelerated response, for early cessation of bacterial growth and for establishment of an IL-17-producing CD4+ T cell population in the lung. The recall response of the IL-17-producing CD4+ T cell population occurred concurrently with expression of the chemokines CXCL9, CXCL10 and CXCL11. Depletion of IL-17 during challenge reduced the chemokine expression and accumulation of CD4+ T cells producing interferon-γ in the lung. We propose that vaccination induces IL-17-producing CD4+ T cells that populate the lung and, after challenge, trigger the production of chemokines that recruit CD4+ T cells producing interferon-γ, which ultimately restrict bacterial growth.

Reciprocal regulation of polarized cytokine production by effector B and T cells

Although B cells produce cytokines it is not known whether B cells can differentiate into effector subsets that secrete polarized arrays of cytokines. We have identified two populations of “effector” B cells (Be1 and Be2) that produce distinct patterns of cytokines depending on the cytokine environment in which the cells were stimulated during their primary encounter with antigen and T cells. These effector B cell subsets subsequently regulate the differentiation of naïve CD4+ T cells to TH1 and TH2 cells through production of polarizing cytokines such as interleukin 4 and interferon γ. In addition, Be1 and Be2 cells could be identified in animals that were infected with pathogens that preferentially induce a Type 1 or Type 2 immune response. Together these results suggest that, in addition to their well defined role in antibody production, B cells may regulate immune responses to infectious pathogens through their production of cytokines.

Expanding roles for CD4+ T cells in immunity to viruses

Viral pathogens often induce strong effector CD4+ T cell responses that are best known for their ability to help B cell and CD8+ T cell responses. However, recent studies have uncovered additional roles for CD4+ T cells, some of which are independent of other lymphocytes, and have described previously unappreciated functions for memory CD4+ T cells in immunity to viruses. Here, we review the full range of antiviral functions of CD4+ T cells, discussing the activities of these cells in helping other lymphocytes and in inducing innate immune responses, as well as their direct antiviral roles. We suggest that all of these functions of CD4+ T cells are integrated to provide highly effective immune protection against viral pathogens.

CD4 Effector T Cell Subsets in the Response to Influenza Heterogeneity, Migration, and Function

The immune response of naive CD4 T cells to influenza virus is initiated in the draining lymph nodes and spleen, and only after effectors are generated do antigen-specific cells migrate to the lung which is the site of infection. The effector cells generated in secondary organs appear as multiple subsets which are a heterogeneous continuum of cells in terms of number of cell divisions, phenotype and function. The effector cells that migrate to the lung constitute the more differentiated of the total responding population, characterized by many cell divisions, loss of CD62L, down-regulation of CCR7, stable expression of CD44 and CD49d, and transient expression of CCR5 and CD25. These cells also secrete high levels of interferon γ and reduced levels of interleukin 2 relative to those in the secondary lymphoid organs. The response declines rapidly in parallel with viral clearance, but a spectrum of resting cell subsets reflecting the pattern at the peak of response is retained, suggesting that heterogeneous effector populations may give rise to corresponding memory populations. These results reveal a complex response, not an all-or-none one, which results in multiple effector phenotypes and implies that effector cells and the memory cells derived from them can display a broad spectrum of functional potentials.

IL-7 Promotes the Transition of CD4 Effectors to Persistent Memory Cells

After transfer to adoptive hosts, in vitro–generated CD4 effectors can become long-lived memory cells, but the factors regulating this transition are unknown. We find that low doses of interleukin (IL) 7 enhance survival of effectors in vitro without driving their division. When in vitro–generated effectors are transferred to normal intact adoptive hosts, they survive and rapidly become small resting cells with a memory phenotype. CD4 effectors generated from wild-type versus IL-7 receptor−/− mice were transferred to adoptive hosts, including intact mice and those deficient in IL-7. In each case, the response to IL-7 was critical for good recovery of donor cells after 5–7 d. Recovery was also IL-7–dependent in Class II hosts where division was minimal. Blocking antibodies to IL-7 dramatically decreased short-term recovery of transferred effectors in vivo without affecting their division. These data indicate that IL-7 plays a critical role in promoting memory CD4 T cell generation by providing survival signals, which allow effectors to successfully become resting memory cells.

Generation and in vivo persistence of polarized Th1 and Th2 memory cells

In vitro generated CD4 cell lines and effectors often produce either Th1 or Th2 cytokines, but stimulation of resting CD4 cells from animals leads to production mostly of IL-2. To determine whether polarization of CD4 effector cells results in development of polarized memory, I generated antigen-specific Th1 and Th2 effectors in vitro and transferred them to adoptive hosts. The effectors gave rise to long-lived populations of CD4 T cells with the phenotype of resting memory cells. Recovered cells responded vigorously to specific antigen, producing a pattern of cytokines closely related to that of the transferred effectors. Thus, encounter with a peptide antigen and directive cytokines at the initiation of culture can lead to generation of both effector and long-lived memory cell populations that produce restricted patterns of cytokines.

Tc17, a Unique Subset of CD8 T Cells That Can Protect against Lethal Influenza Challenge

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.

Qualitative Changes Accompany Memory T Cell Generation: Faster, More Effective Responses at Lower Doses of Antigen

The generation of memory T cells is critically important for rapid clearance and neutralization of pathogens encountered previously by the immune system. We have studied the kinetics of response and Ag dose requirements for proliferation and cytokine secretion of CD4+ memory T cells to examine whether there are qualitative changes which might lead to improved immunity. TCR Tg CD4+ T cells were primed in vitro and transferred into T cell-deficient hosts. After 6 or more weeks, the persisting T cells were exclusively small resting cells with a memory phenotype: CD44high CD62L+/− CD25−. Memory CD4 T cells showed a similar pattern of response as naive cells to peptide analogues with similar Ag dose requirements for IL-2 secretion. However, memory cells (derived from both Th2 and Th1 effectors) displayed faster kinetics of cytokine secretion, cell division, and proliferation, enhanced proliferation in response to low doses of Ag or peptide analogues, and production of IL-4, IL-5, and IFN-γ. These results suggest there is a much more efficient response of CD4 memory T cells to Ag re-exposure and that the expanded functional capacity of memory cells will promote a rapid development of effector functions, providing more rapid and effective immunity.

From Naive to Memory T Cells

Specific immune protection against pathogens is achieved by the development of a diverse set of lymphoid cells whose products can target the organisms for destruction and clearance: A spectrum of highly specific antibodies (Ab)# produced by the progeny of B lymphocytes and T cells with receptors for antigenic determinants of the pathogen, seen as peptides complexed with major histocompatibility complex (MHC) molecules. The Ab can bind to the pathogen while it is pre.sent extracellularty, while the T cells can detect infected cells. When a pathogen infects an animal that has not previously encountered a similar organism, a complex, interdependent series of cellular responses occurs which can generate an effective specific immune response only after a period of 4-7 days. During this time the protection against the infectious organism is primarily limited to the largely nonspecific components of innate immunity and the consequence for the host can be the particular illness associated with the pathogen. The disease is a combination of the direct pathological effects of the infection, coupled with effects of the innate immune response, which can also be injurious. These innate protective mechanisms, which include natural killer cells and induction of granulocytes and macrophage products, are often only partially effective allowing the pathogen to continue replicating and/or infecting cells. The specific effecter response that develops must be sufficiently robust to deal with a large load of infectious organisms. Achieving destruction of the pathogen may require not only initial development of T and B effector populations, but also their further expansion to a size that is sufficient to generate enough Ab and

Interleukin 12p40 is required for dendritic cell migration and T cell priming after Mycobacterium tuberculosis infection

Migration of dendritic cells (DCs) to the draining lymph node (DLN) is required for the activation of naive T cells. We show here that migration of DCs from the lung to the DLN after Mycobacterium tuberculosis (Mtb) exposure is defective in mice lacking interleukin (IL)-12p40. This defect compromises the ability of IL-12p40–deficient DCs to activate naive T cells in vivo; however, DCs that express IL-12p40 alone can activate naive T cells. Treatment of IL-12p40–deficient DCs with IL-12p40 homodimer (IL-12(p40)2) restores Mtb-induced DC migration and the ability of IL-12p40–deficient DCs to activate naive T cells. These data define a novel and fundamental role for IL-12p40 in the pathogen-induced activation of pulmonary DCs.