T Cell Exhaustion: Chronic Stimulation and the Exhaustion Hierarchy (2D)

Chronic Stimulation

Antigen stimulation rate 1.0×

Simulation speed 1.0×

Checkpoint Blockade (anti-PD-1)

Blockade dose 0%

Blockade only reinvigorates the cell while it is still progenitor-exhausted (below the epigenetic fixation point). Once terminally exhausted, the drug has no effect.

Cell State

Stimulation load S

State

Effector

Inhibitory Receptors

Tumor burden

100%

Point of no return

70% S

How it works

Under persistent antigen (chronic infection or a growing tumor), the CD8⁺ T cell accumulates a stimulation load S(t) that rises with the local antigen/stimulation rate r and decays slowly between encounters:

dS/dt = r · (1 − S) − k_decay · S
Exhaustion score  E = S  (0 = naive/effector, 1 = fully exhausted)
Inhibitory receptors (PD-1, LAG-3, TIM-3, TIGIT) each turn on once E crosses its own threshold, so co-expression count rises monotonically with E.

Effector functions are lost in a fixed biological order as E rises (Wherry & Kaech's exhaustion hierarchy) — the most metabolically "expensive" functions go first:

IL-2 production   lost first,  threshold ≈ 0.25
TNF-α production  lost second, threshold ≈ 0.45
IFN-γ production  lost third,  threshold ≈ 0.65
Cytotoxicity      lost last,   threshold ≈ 0.85

Once the cell crosses a fixation threshold (E ≈ 0.7) it becomes terminally exhausted: chromatin is epigenetically locked and checkpoint blockade cannot restore function. Below that line the cell is still progenitor-exhausted (PD-1^int, TCF-1⁺) — blocking PD-1 lowers the effective inhibitory signal and measurably slows/reverses S, which is exactly why checkpoint inhibitors work early but stop helping once exhaustion has been progressing for a long time.

Antigen stimulation rate — how aggressively the tumor/infection drives the T cell; higher rate pushes S up faster.
Checkpoint blockade dose — multiplies the decay term while the cell is still progenitor-exhausted (0% = no drug, 100% = full PD-1 blockade).
Tumor burden — driven down by cytotoxic killing that still functions; it grows back once the cell exhausts.

This is a simplified single-cell ODE model, not a full agent-based immune simulation, but the thresholds, order of functional loss, and progenitor/terminal split reflect the real experimental literature on LCMV chronic infection and tumor-infiltrating lymphocytes.

t = 0.0 (arb. days)

Radar: current functional capacity

Effector T cell Progenitor-exhausted Terminally exhausted Tumor cell