Antonio J. Pagán

Acute Gastrointestinal Infection Induces Long-Lived Microbiota-Specific T Cell Responses

Recognizing Escaped Commensals In order to coexist peacefully, the billions of bacteria in our gut and our immune system have reached a détente. An intestinal mucosal firewall exists, so bacteria remain localized to the gut, where the immune system is tightly regulated so that these bacteria are tolerated. Enteric infections, however, lead to a breach in this mucosal firewall, resulting in exposure of the peripheral immune system to the intestinal bacterial contents. What is the result? Using oral Toxoplasma gondii infection in mice, Hand et al. (p. 1553, published online 23 August) show that, besides the T. gondii–specific T cell response, a commensal bacteria–specific T cell response is elicited. The CD4+ T cell–specific response was tracked to a commensal-derived flagellin, and these T cells expanded after T. gondii infection and formed long-lived memory cells able to respond to subsequent challenges. Thus, enteric infections can lead to the formation of commensal bacteria–specific, long-lived memory T cells that reside throughout the body—which may play a role in intestinal pathologies such as inflammatory bowel disease. Enteric infections induce lasting adaptive immunity against commensal bacteria that may play a role in intestinal problems. The mammalian gastrointestinal tract contains a large and diverse population of commensal bacteria and is also one of the primary sites of exposure to pathogens. How the immune system perceives commensals in the context of mucosal infection is unclear. Here, we show that during a gastrointestinal infection, tolerance to commensals is lost, and microbiota-specific T cells are activated and differentiate to inflammatory effector cells. Furthermore, these T cells go on to form memory cells that are phenotypically and functionally consistent with pathogen-specific T cells. Our results suggest that during a gastrointestinal infection, the immune response to commensals parallels the immune response against pathogenic microbes and that adaptive responses against commensals are an integral component of mucosal immunity.

Different routes of bacterial infection induce long-lived T H 1 memory cells and short-lived T H 17 cells

We used a sensitive method based on tetramers of peptide and major histocompatibility complex II (pMHCII) to determine whether CD4+ memory T cells resemble the T helper type 1 (TH1) and interleukin 17 (IL-17)-producing T helper (TH17) subsets described in vitro. Intravenous or intranasal infection with Listeria monocytogenes induced pMHCII-specific CD4+ naive T cells to proliferate and produce effector cells, about 10% of which resembled TH1 or TH17 cells, respectively. TH1 cells were also present among the memory cells that survived 3 months after infection, whereas TH17 cells disappeared. The short lifespan of TH17 cells was associated with small amounts of the antiapoptotic protein Bcl-2, the IL-15 receptor and the receptor CD27, and little homeostatic proliferation. These results suggest that TH1 cells induced by intravenous infection are more efficient at entering the memory pool than are TH17 cells induced by intranasal infection.

Single naive CD4+ T cells from a diverse repertoire produce different effector cell types during infection.

A naive CD4(+) T cell population specific for a microbial peptide:major histocompatibility complex II ligand (p:MHCII) typically consists of about 100 cells, each with a different T cell receptor (TCR). Following infection, this population produces a consistent ratio of effector cells that activate microbicidal functions of macrophages or help B cells make antibodies. We studied the mechanism that underlies this division of labor by tracking the progeny of single naive T cells. Different naive cells produced distinct ratios of macrophage and B cell helpers but yielded the characteristic ratio when averaged together. The effector cell pattern produced by a given naive cell correlated with the TCR-p:MHCII dwell time or the amount of p:MHCII. Thus, the consistent production of effector cell subsets by a polyclonal population of naive cells results from averaging the diverse behaviors of individual clones, which are instructed in part by the strength of TCR signaling.

Tracking epitope-specific T cells.

The tracking of antigen-specific T cells in vivo is a useful approach for the study of the adaptive immune response. This protocol describes how populations of T cells specific for a given peptide–major histocompatibility complex (pMHC) epitope can be tracked based solely on T-cell receptor (TCR) specificity as opposed to other indirect methods based on function. The methodology involves the adoptive transfer of TCR transgenic T cells with defined epitope specificity into histocompatible mice and the subsequent detection of these cells through the use of congenic or clonotypic markers. Alternatively, endogenous epitope-specific T cells can be tracked directly through the use of pMHC tetramers. Using magnetic bead-based enrichment and advanced multiparameter flow cytometry, populations as small as five epitope-specific T cells can be detected from the peripheral lymphoid organs of a mouse. The adoptive transfer procedure can be completed within 3 h, whereas analysis of epitope-specific cells from mice can be completed within 6 h.

Cutting Edge: CD28 and c-Rel–Dependent Pathways Initiate Regulatory T Cell Development

Regulatory T cell (Treg) development proceeds via a two-step process in which naive CD4+ thymocytes are first converted into CD4+CD25+CD122+GITR+Foxp3− Treg progenitors, followed by a second step in which IL-2 converts these Treg progenitors into CD4+Foxp3+ Tregs. The costimulatory molecule CD28 is required for efficient Treg development. However, the stage at which CD28 affects Treg development remains undefined. In this article, we demonstrate that Cd28−/− mice lack Treg progenitors. Furthermore, the P187YAP motif in the cytoplasmic tail of CD28, which links CD28 to Lck activation, is required for this process. In contrast, the Y170MNM motif, which links CD28 to PI3K activation, is not required for Treg progenitor development. Finally, the CD28/Lck pathway was shown to activate the NF-κB family of transcription factors. We demonstrate that c-Rel, but not NF-κB1, promotes the development of Treg progenitors. Thus, a CD28/c-Rel–dependent pathway is involved in initiating Treg development.

Control of α4β7 Integrin Expression and CD4 T Cell Homing by the β1 Integrin Subunit

The α4β7 integrin promotes homing of T cells to intestinal sites. The α4 integrin subunit that pairs with β7 integrin can also pair with β1 integrin. In this paper, we show that the preferential pairing of β1 integrin with α4 integrin regulates the expression of α4β7 on T cells. In the absence of β1 integrin, naive mouse CD4 T cells have increased α4β7 expression, resulting in increased adhesion to mucosal addressin cell adhesion molecule-1 and enhanced homing to Peyer’s patches (PP). In a reciprocal manner, overexpression of β1 integrin causes the loss of α4β7 expression and decreased homing to PP. A similar upregulation of β1 integrin and suppression of α4β7 expression occurs rapidly after CD4 T cell activation. β1 integrin thus dominates β7 integrin for α4 integrin pairing, thereby controlling the abundance of unpaired α4 integrin. Increasing the abundance of α4 integrin relative to β1 integrin is critical to retinoic acid-mediated expression of α4β7 integrin during T cell activation. In the absence of β1 integrin, endogenous Ag-specific CD4 T cells uniformly express high levels of α4β7 after Listeria monocytogenes infection. The resulting β1-deficient early memory T cells have decreased localization to the bone marrow and enhanced localization to PP after infection. Thus, the preferential association of β1 integrin with α4 integrin suppresses α4β7 integrin expression and regulates the localization of memory CD4 T cells.

Chronic Parasitic Infection Maintains High Frequencies of Short-Lived Ly6C+CD4+ Effector T Cells That Are Required for Protection against Re-infection

In contrast to the ability of long-lived CD8+ memory T cells to mediate protection against systemic viral infections, the relationship between CD4+ T cell memory and acquired resistance against infectious pathogens remains poorly defined. This is especially true for T helper 1 (Th1) concomitant immunity, in which protection against reinfection coincides with a persisting primary infection. In these situations, pre-existing effector CD4 T cells generated by ongoing chronic infection, not memory cells, may be essential for protection against reinfection. We present a systematic study of the tissue homing properties, functionality, and life span of subsets of memory and effector CD4 T cells activated in the setting of chronic Leishmania major infection in resistant C57Bl/6 mice. We found that pre-existing, CD44+CD62L−T-bet+Ly6C+ effector (TEFF) cells that are short-lived in the absence of infection and are not derived from memory cells reactivated by secondary challenge, mediate concomitant immunity. Upon adoptive transfer and challenge, non-dividing Ly6C+ TEFF cells preferentially homed to the skin, released IFN-γ, and conferred protection as compared to CD44+CD62L−Ly6C− effector memory or CD44+CD62L+Ly6C− central memory cells. During chronic infection, Ly6C+ TEFF cells were maintained at high frequencies via reactivation of TCM and the TEFF themselves. The lack of effective vaccines for many chronic diseases may be because protection against infectious challenge requires the maintenance of pre-existing TEFF cells, and is therefore not amenable to conventional, memory inducing, vaccination strategies.

Tracking antigen-specific CD4+ T cells throughout the course of chronic Leishmania major infection in resistant mice

Primary Leishmania major infection typically produces cutaneous lesions that not only heal but also harbor persistent parasites. While the opposing roles of CD4+ T‐cell‐derived IFN‐γ and IL‐10 in promoting parasite killing and persistence have been well established, how these responses develop from naïve precursors has not been directly monitored throughout the course of infection. We used peptide:Major Histocompatibility Complex class II (pMHCII) tetramers to investigate the endogenous, parasite‐specific primary CD4+ T‐cell response to L. major in mice resistant to infection. Maximal frequencies of IFN‐γ+ CD4+ T cells were observed in the spleen and infected ears within a month after infection and were maintained into the chronic phase. In contrast, peak frequencies of IL‐10+ CD4+ T cells emerged within 2 weeks of infection, persisted into the chronic phase, and accumulated in the infected ears but not the spleen, via a process that depended on local antigen presentation. T helper type‐1 (Th1) cells, not Foxp3+ regulatory T cells, were the chief producers of IL‐10 and were not exhausted. Therefore, tracking antigen‐specific CD4+ T cells revealed that IL‐10 production by Th1 cells is not due to persistent T‐cell antigen receptor stimulation, but rather driven by early antigen encounter at the site of infection.

Most microbe-specific naïve CD4+ T cells produce memory cells during infection

All T cells can remember One of the hallmarks of adaptive immunity is that T and B lymphocytes “remember” previous infections, protecting the host from subsequent infections. When T cells respond to a pathogen, they proliferate, and a fraction of their progeny goes on to form long-lived memory cells. It is not clear whether all of the T cell clones that respond to the initial infection have the potential to form memory T cells. Tubo et al. used a single-cell adoptive transfer model in mice to answer this question. Nearly all T cell clones produced memory cells, which suggests that breadth is probably an important component of immunological memory. Science, this issue p. 511 Nearly every immunological T cell has the potential to give rise to memory cells. Infection elicits CD4+ memory T lymphocytes that participate in protective immunity. Although memory cells are the progeny of naïve T cells, it is unclear that all naïve cells from a polyclonal repertoire have memory cell potential. Using a single-cell adoptive transfer and spleen biopsy method, we found that in mice, essentially all microbe-specific naïve cells produced memory cells during infection. Different clonal memory cell populations had different B cell or macrophage helper compositions that matched effector cell populations generated much earlier in the response. Thus, each microbe-specific naïve CD4+ T cell produces a distinctive ratio of effector cell types early in the immune response that is maintained as some cells in the clonal population become memory cells.

CD28 Promotes CD4+ T Cell Clonal Expansion during Infection Independently of Its YMNM and PYAP Motifs

CD28 is required for maximal proliferation of CD4+ T cells stimulated through their TCRs. Two sites within the cytoplasmic tail of CD28, a YMNM sequence that recruits PI3K and activates NF-κB and a PYAP sequence that recruits Lck, are candidates as transducers of the signals responsible for these biological effects. We tested this proposition by tracking polyclonal peptide:MHCII-specific CD4+ T cells in vivo in mice with mutations in these sites. Mice lacking CD28 or its cytoplasmic tail had the same number of naive T cells specific for a peptide:MHCII ligand as wild-type mice. However, the mutant cells produced one tenth as many effector and memory cells as wild-type T cells after infection with bacteria expressing the antigenic peptide. Remarkably, T cells with a mutated PI3K binding site, a mutated PYAP site, or both mutations proliferated to the same extent as wild-type T cells. The only observed defect was that T cells with a mutated PYAP or Y170F site proliferated even more weakly in response to peptide without adjuvant than wild-type T cells. These results show that CD28 enhances T cell proliferation during bacterial infection by signals emanating from undiscovered sites in the cytoplasmic tail.