Regulation of CD4+ T-cell contraction during pathogen challenge

Abstract

Signals orchestrating productive CD4+ T-cell responses are well documented; however, the regulation of contraction of CD4+ T-cell effector populations following the resolution of primary immune responses is not well understood. While distinct mechanisms of T-cell death have been defined, the relative importance of discrete death pathways during the termination of immune responses in vivo remains unclear. Here, we review the current understanding of cell-intrinsic and -extrinsic variables that regulate contraction of CD4+ T-cell effector populations through multiple pathways that operate both initially during T-cell priming and later during the effector phase. We discuss the relative importance of antigen-dependent and -independent mechanisms of CD4+ T-cell contraction during in vivo responses, with a special emphasis on influenza virus infection. In this model, we highlight the roles of greater differentiation and presence in the lung of CD4+ effector T cells, as well as their polarization to particular T-helper subsets, in maximizing contraction. We also discuss the role of autocrine interleukin-2 in limiting the extent of contraction, and we point out that these same factors regulate contraction during secondary CD4+ T-cell responses.

Figures

Fig. 1. T-cell contraction following influenza infection

Following maximal effector responses in the lung, an acute contraction phase (yellow highlighted) occurs in the lung that is coincident with viral clearance.

Fig. 2. CD4+ T-cell contraction following influenza infection

Naive TCR Tg CD4+ T cells recognizing influenza (HNT) were transferred to BALB/c hosts then infected with a sublethal dose of virus. Donor CD4+ T-cell numbers were determined between 7 and 30 days post infection. Data presented on a Log2 scale to emphasize the early acute contraction in all sites, but more prolonged contraction seen in the lung.

Fig. 3. Lung-resident effector CD4+ T cells are most susceptible to AICD during influenza infection

Naive HNT CD4+ T cells were transferred to naive hosts, which were then inflected with influenza. At 7 (shaded) and 14 (red line) days post infection, donor CD4+ T cells were recovered from the lung or draining lymph nodes (DLN). Cells were then restimulated with anti-CD3 for 20 hours and stained with 7-AAD to detect apoptotic cells.

Fig. 4. Effector CD4+ T cells rapidly loose susceptibility to AICD

(A) In vitro-generated Th1 effectors or effectors that rested for 3 days in vivo were either left unstimulated or restimulated for 20 h with peptide-pulsed antigen-presenting cells (APCs). Significantly fewer rested effectors underwent AICD, as determined by assessing a variety of markers associated with apoptotic death. (B) Naive HNT CD4+ T cells were compared to activated Th1 and Th2 effectors (generated in vitro), as well as memory cells (generated following adoptive transfer of in vitro effectors into antigen-free hosts). All populations were restimulated in vitro with antigen and APC, and CD4+ T-cell apoptosis in responding cultures was assessed by TUNEL staining.

Fig. 5. IFNγ does not appreciably impact contraction of responding CD4+ T cells during influenza infection

Naive wildtype (WT) HNT CD4+ T cells were transferred to WT BALB/c hosts or naive IFNγ KO HNT cells transferred to IFNγ KO hosts. Mice were then inflected with a sublethal dose of influenza. Donor CD4+ T-cell numbers in the lung were determined at 7 and 14 days post-infection.

Fig. 6. Th-polarization impacts antigen-independent contraction of CD4+ T-cell effectors

Naive HNT cells were cultured with antigen and antigen presenting cells under Th0, Th1, Th2, Th17, or Treg polarizing conditions in vitro. After 4 days, well-polarized effectors were rested for three days in the absence of antigen and cytokines and the number of live cells determined (depicted as percentage of effector cells).

Fig. 7. In vitro contraction of Th1-polarized effector populations is regulated by effector molecules

Naive WT, IFNγ KO, or perforin KO HNT CD4+ T cells cultured under Th1-polarizing conditions in vitro. After 4 days, resulting effectors were rested for three days in the absence of antigen and cytokines and the number of live cells determined (depicted as percentage of effector cells).

Fig. 8. IL-2 regulates contraction of memory-effectors

Naive WT or IL-2 KO CD4+ T cells were cultured for 4 days under Th1-polarizing, including exogenous IL-2 conditions and then rested for 3 days in the absence of antigen and cytokines to generate memory. WT and IL-2 KO memory cells were transferred to WT hosts, then inflected with influenza, and donor CD4+ T-cell numbers were determined between.

Fig. 9. Signals impacting contraction of CD4+ T cells in vivo

In the schematic, we point out key roles for factors that influence contraction at different stages of response (red arrows denote increased contraction, and blue arrows denote reduced contraction). First, in regulating the programming of effectors to susceptibility to contraction by inducing maximal differentiation and migration to sites of infection (to the lung in the influenza model). Second, at the effector stage where availability of antigen, and autocrine and paracrine cytokines can further modulate the extent of death and degree of contraction, determining what fraction of the population survives to memory.