Natalizumab: More Than a Traffic Cop—It Sends Signals Too

Natalizumab (Tysabri) is famous for one big thing: it blocks immune cells from squeezing into the brain and spinal cord by targeting the α4 subunit of the integrin VLA-4 (CD49d/CD29). That “traffic-stopper” effect is why it’s so effective in relapsing–remitting multiple sclerosis (MS). But a PLOS ONE study adds a twist: the drug can also talk back to T cells—nudging them toward a more inflammatory state in some people.

Why look beyond adhesion?
Clinicians have long noticed two thorny issues around natalizumab: rare cases of PML (a JC virus–driven brain infection) during therapy, and, after the drug is stopped or rapidly cleared, bursts of inflammatory activity (rebound or IRIS). Both suggest that what happens to immune cells outside the CNS matters, too. The authors asked a simple question with big implications: does binding CD49d with natalizumab actively signal inside T cells, not just block their adhesion?

What the researchers did—in people and in cells
Ex vivo human T cells: Primary CD4⁺ T cells from healthy donors were stimulated under standard conditions, with or without natalizumab at clinically relevant concentrations. The team profiled gene expression (microarrays, qRT-PCR), measured cytokines by intracellular flow, and probed signaling pathways by Western blot.

First-dose MS patients: Nine individuals with RRMS were sampled immediately before and 24 hours after their very first infusion; their PBMCs were analyzed without adding extra drug in vitro, so any changes reflect in-vivo exposure.

The headline: a mild but real “outside-in” costimulus
Cytokines tick up. In activated CD4⁺ T cells, natalizumab modestly increased transcripts and/or intracellular protein for IL-2, IFN-γ, and IL-17—a profile consistent with Th1/Th17-leaning responses. IL-10 trended down at the RNA level. Pathway analysis flagged “cytokine–cytokine receptor interaction” among the most enriched sets in natalizumab-treated cells.

MAPK/ERK lights up quickly. Within hours, natalizumab triggered rapid ERK phosphorylation (and context-dependent JNK), echoing canonical integrin signaling. Notably, NFAT nuclear translocation didn’t rise, suggesting the drug mainly boosts the AP-1 side of the NFAT/AP-1 cytokine engine via MAPK.

The receptor itself is pulled inside. CD49d surface density dropped within hours; imaging showed intracellular speckles of CD49d consistent with internalization and degradation, not shedding (MMP-2/9 in supernatants fell rather than rose). Functionally, the CD49d^hi subset—already the most cytokine-productive—was preferentially depleted from the surface pool after drug engagement.

What changed in patients after a single infusion?
Twenty-four hours after first-dose natalizumab, many patients’ PBMCs showed:

Lower surface CD49d, amplified by TCR restimulation in vitro, mirroring the ex vivo findings.

Higher IL-2, IL-17, and/or IFN-γ in a subset of patients on intracellular staining; IL-2 and IL-17 increases were strongly correlated within individuals. Supernatants from restimulated cells also showed higher IL-2 and IFN-γ, and a rise in IL-12/IL-23p40, hinting that antigen-presenting cells may join this pro-inflammatory chorus. The effect was heterogeneous—some people didn’t shift, while others clearly did.

A working model
TCR engagement provides the classic inside-out “prime” for integrins.

Natalizumab binding delivers an outside-in nudge through α4 integrin, boosting MAPK/ERK, which strengthens AP-1–dependent cytokine programs (with NFAT largely unchanged). Surface CD49d drops via internalization, reducing trafficking—but meanwhile, peripheral T cells can become slightly more inflammatory. In susceptible individuals, that could stockpile Th1/Th17-skewed cells in blood. When natalizumab is stopped or rapidly cleared (e.g., plasma exchange), those cells may re-enter the CNS, helping explain rebound and IRIS phenomena in some cases.

Practical implications (and caveats)
Not everyone responds the same. The cytokine boost was modest and inter-individual—a key nuance. It’s a supporting signal, not a wholesale reprogramming.

Mechanistic plausibility for clinical observations. The data don’t prove causation for rebound or PML-IRIS, but they sketch a biologically coherent path from α4 engagement → ERK/AP-1 → Th1/Th17 tone, aligning with reports of increased peripheral pro-inflammatory T cells during treatment.

Drug design and monitoring. Next-gen α4 blockers (or dosing/washout strategies) might be assessed for signaling potency, not just adhesion blockade. Short-term biomarkers—IL-2/IL-17/IFN-γ in T cells, pERK responses, CD49d dynamics—could help identify “signal-sensitive” patients.

Limitations to keep in mind
Small first-dose cohort, short follow-up (24–48 h), and largely ex vivo readouts mean we can’t directly tie these immune shifts to clinical outcomes. The signaling changes are mild and context-dependent (strongest under physiologic-like TCR stimulation), which fits biology but complicates prediction.

The takeaway
Natalizumab’s superpower is still blocking entry—but this work shows it can also whisper to T cells through α4 integrin, subtly amplifying Th1/Th17 programs via ERK in some people. That dual nature—anti-trafficking plus pro-inflammatory costimulation—may help explain why stopping therapy occasionally unleashes a storm. As we refine MS treatment, paying attention to both “where cells can go” and “what signals they’re hearing” will be essential.

Disclaimer: This blog post is based on the provided research article and is intended for informational purposes only. It is not intended to provide medical advice. Please consult with a healthcare professional for any health concerns.

References:
Kemmerer, C. L., Pernpeintner, V., Ruschil, C., Abdelhak, A., Scholl, M., Ziemann, U., ... & Kowarik, M. C. (2020). Differential effects of disease modifying drugs on peripheral blood B cell subsets: A cross sectional study in multiple sclerosis patients treated with interferon-β, glatiramer acetate, dimethyl fumarate, fingolimod or natalizumab. PLoS One, 15(7), e0235449.