Tonic Signals: Why Do Lymphocytes Bother?

Abstract

Since the 1990s it has been known that B and T lymphocytes exhibit low-level, constitutive signaling in the basal state (tonic signaling). These lymphocytes display a range of affinity for self, which in turn generates a range of tonic signaling. Surprisingly, what signaling pathways are active in the basal state and the functional relevance of the observed tonic signaling heterogeneity remain open questions today. Here we summarize what is known about the mechanistic and functional details of tonic signaling. We highlight recent advances that have increased our understanding of how the amount of tonic signal impacts immune function, describing novel tools that have moved the field forward and toward a molecular understanding of tonic signaling.

Keywords: B lymphocyte; T lymphocyte; tonic signaling.

Figure 1. Receptor-ablation studies provide insights into the role of tonic signals in B- and T- lymphocytes

When the BCR is ablated from B cells, they rapidly die within 3–6 days. Additional experiments uncovered a role for IgD-PI3K signaling and a potential role for antigen exposure in this tonic survival signal. Ablation of the TCR from CD8⁺, CD4⁺, or CD4⁺ regulatory T cells revealed a maintenance and functional role, and not a survival role, for the TCR in tonic signaling. Of note, while TCR ablation affects the functional capacities of regulatory T cells, it does not have an effect on Foxp3 expression. The half-life of receptor-negative cells, as well as what is known about the mechanism of this tonic signal, are outlined below each cell type. Receptor-positive CD8⁺ T cells have a half-life of 162 days, and receptor-positive CD4⁺ cells have a half-life of 78 days.

Figure 2. T cells receive tonic signals in lymphoid organs but not in the bloodstream

T cells in peripheral lymphoid organs (PLOs) are in a surveying mode and make transient contacts with self-pMHC. These contacts generate a low-level, constitutive signal (“tonic signal”). In contrast, cells in the bloodstream do not make these contacts with self, and are deprived of tonic signals. It is unknown whether the tonic signal “resets” as lymphocytes transit from the PLOs to the blood and back again.

Figure 3. CD5 and Nur77-eGFP report the level of tonic signaling in B and T cells

CD5 is a surface glycoprotein expressed on T cells and the level of CD5 corresponds to the level of tonic signal - and thus the level of affinity for self by a particular T cell. CD5 is only expressed on a subset of B cells called B-1a B cells, but does not appear to mark the level of tonic signaling in B cells. Nur77 (Nr4a1) is an immediate-early gene that gets induced proportionally to the level of signal in both T- and B- cells. BAC-transgenic mice were generated in which eGFP is under control of the Nur77 regulatory region. The level of GFP reads-out the level of tonic signal a B or T cell receives.

Figure 4. Inducible deletion of LAT in T cells reveals a role for tonic signals in controlling gene expression

Cartoon of genetic mouse model with exons 7 to 11 of LAT flanked by LoxP recombination sites. Crossing these mice to Cre-ER allows for inducible deletion of LAT upon administration of tamoxifen to release the Cre recombinase from the estrogen receptor (ER). Studies using this inducible model revealed a role for tonic TCR-LAT-PLCγ signals in controlling gene expression in CD4 T cells.

Figure 5. The Rasgrp1^Anaef variant generates increased tonic mTORC1-S6 signals that drive T cell autoreactivity

Analysis of mice with a point-mutated Rasgrp1 variant (Rasgrp1^Anaef) revealed that increased tonic Rasgrp1-mTORC1 signals drive elevated CD44 expression on T cells, which can be reduced by in vivo administration of the mTOR inhibitor rapamycin.

Figure 6. Hypothesis that single nucleotide variants (SNVs) in signaling genes impact tonic signals and cause T cell autoimmunity

Several mouse models have been described in which single point mutations that lead to amino acid changes (SNVs) in signaling molecules can lead to autoimmune features. The LAT and Rasgrp1 models described reveal that these SNVs can alter tonic signaling in T cells. Since every person inherits 12,000 SNVs on average, it is likely that some of these SNVs impact tonic signaling in human T cells and contribute to autoimmunity.