The disclosure problem: Most commercial MSC exosome suppliers do not disclose the passage number at which cells were harvested. This is the single most consequential piece of missing information for evaluating whether a preparation reflects the therapeutic MSC secretome described in published literature.

What Is a Passage?

A passage — also called a subculture — occurs when a confluent culture of MSCs is enzymatically detached from the culture surface (or dissociated from a 3D format), counted, and replated at a lower density to allow continued proliferation. Each passage resets the density-inhibition clock but accumulates population doublings toward the cell's finite replicative capacity.

Passage numbering begins at P0 (the primary isolation from tissue) or P1 (the first subculture after primary isolation). Convention varies by laboratory, but the cumulative number of passages — and the cumulative number of population doublings — increases monotonically with culture time. MSCs expanded from umbilical cord tissue can typically achieve P12-P15 before reaching irreversible replicative senescence under standard culture conditions.

The passage number at which cells are harvested for exosome collection is not a manufacturing detail. It is the single most important determinant of whether the harvested cells are expressing the therapeutic secretome described in the peer-reviewed literature, or whether they have transitioned into a senescence-associated secretory phenotype that produces the opposite effect.

Replicative Senescence and the Hayflick Limit

In 1961, Leonard Hayflick demonstrated that normal human diploid cells have a finite replicative lifespan — approximately 50-70 population doublings for embryonic cells. This became known as the Hayflick limit. The mechanistic basis was elucidated over subsequent decades: it is primarily driven by telomere shortening. With each cell division, 50-200 base pairs of telomeric DNA are lost from chromosome ends due to the end-replication problem. When telomeres shorten below a critical threshold, the cell activates a DNA damage response mediated by ATM/ATR kinases and p53 that drives permanent cell cycle arrest — replicative senescence.

MSCs have relatively long telomeres at isolation and can achieve more population doublings than many other somatic cell types before reaching Hayflick-limit senescence. However, passage-associated stress accumulates through mechanisms beyond telomere shortening, including oxidative damage accumulation, epigenetic drift, and oncogene-induced senescence pathways. This means that functional senescence — including SASP emergence — typically precedes full Hayflick-limit senescence in cultured MSCs.

The Passage Trajectory: Four Phases

Phase 1: P0-P1 (Isolation and Primary Culture)

Fresh isolation. Cells are heterogeneous and include fibroblast contaminants, endothelial cells, pericytes, and true MSC progenitors. Passage 1 enriches the plastic-adherent fraction and establishes a more homogeneous MSC population. Not typically used for exosome production due to variable composition.

Phase 2: P2-P4 (Peak Activity Window)

This is the window in which MSCs express full immunomodulatory capacity, maximal trophic factor secretion, and optimal exosome yield and cargo. The following functional characteristics define this phase:

All published peer-reviewed studies demonstrating MSC immunomodulatory function, anti-inflammatory secretome, and therapeutic paracrine effects in preclinical models used cells in this passage range — whether stated explicitly or recoverable from methods sections. When a publication reports an "MSC exosome" result, P2-P4 is the effective reference standard.

Phase 3: P5-P7 (Transition Zone)

Early stress accumulation. Individual cells begin entering senescence, creating a heterogeneous culture with variable function. Cells in this zone still pass all standard MSC identity criteria (ISCT surface markers, tri-lineage differentiation), but functional assays begin showing reduced immunosuppression capacity, declining VEGF and HGF secretion, and the first detectable increases in SA-beta-galactosidase staining and p21 expression. Exosome cargo begins shifting, with reduced miR-146a loading and initial increases in pro-inflammatory miRNA signatures. This zone is increasingly unacceptable for therapeutic-grade production.

Phase 4: P8+ (SASP Phase)

Senescence-associated secretory phenotype is established. The culture contains a substantial population of cells that have entered irreversible growth arrest and are actively secreting a pro-inflammatory, pro-degradative mix of cytokines, growth factors, and matrix metalloproteinases that constitutes SASP. This is not a gradual decline — it represents a qualitative shift in cellular phenotype that alters the entire secretome in a direction opposite to the therapeutic objective.

SASP: What High-Passage MSCs Secrete

Senescence-associated secretory phenotype was characterized by Campisi and colleagues as a stereotyped pro-inflammatory secretory program activated by p38-MAPK, NF-kB, and mTOR signaling downstream of the DNA damage response. In MSCs at P8+, the following SASP components dominate the secretome:

Factor P2-P4 (Therapeutic) P8+ (SASP) Functional Consequence
IL-6 Low basal Strongly elevated Pro-inflammatory cytokine; JAK-STAT3 activation; promotes inflammatory cascade
IL-8 / CXCL8 Low basal Strongly elevated Neutrophil and macrophage recruitment; amplifies local inflammatory response
MMP-1 Low Elevated Interstitial collagenase; matrix degradation; tissue disruption
MMP-3 Low Elevated Stromelysin; broad ECM degradation; disrupts tissue architecture
IGFBP-7 Low Elevated IGF-1 sequestration; reduced pro-survival IGF signaling in neighboring cells; promotes senescence spread
IL-10 Elevated Reduced Lost anti-inflammatory brake; T-reg induction capacity diminished
VEGF Elevated Reduced Lost angiogenic support; HIF-1alpha pathway suppressed
HGF Elevated Significantly reduced Lost anti-apoptotic and anti-fibrotic paracrine signaling
HLA-DR Negative Upregulated Increased immunogenicity; potential for immune recognition and clearance

What High-Passage Exosomes Carry

Exosomes secreted by SASP-positive MSCs carry the SASP cargo. This has been directly demonstrated in proteomic studies of EVs from senescent versus young-passage MSCs:

A preparation of 60 billion EVs from P12 MSCs is not the same product as 60 billion EVs from P3 MSCs. They share a size range and a tetraspanin signature, but their protein and RNA cargo are systematically different — and the P12-derived preparation carries a pro-inflammatory cargo profile that is the opposite of the peer-reviewed therapeutic literature.

HLA-DR Upregulation: Clinical Significance

The ISCT MSC identity criteria require HLA-DR negativity. At low passage (P2-P4), MSCs are HLA-DR negative or express very low surface levels, contributing to their recognized immunoprivileged status. With passage accumulation, HLA-DR surface expression increases — first in a small fraction of the culture, then progressively across the population.

HLA-DR is a MHC class II molecule required for CD4+ T cell antigen presentation. HLA-DR-positive MSCs can present antigen to CD4+ T cells and potentially drive adaptive immune responses. This is clinically significant for any preparation derived from HLA-DR-positive cells, as the preparation may carry HLA-DR on the exosome surface — inherited from the producing cell's plasma membrane — and may trigger T cell responses in allogeneic recipients.

Importantly, a high-passage MSC culture that has become partially HLA-DR positive will still pass the ISCT negativity criterion if the positive fraction remains below the threshold of the testing method. This is one reason why passage number disclosure cannot be replaced by surface marker testing alone.

The Commercial Disclosure Problem

An analysis of commercial MSC exosome products available in the market in 2022-2024 found that fewer than 20% disclosed passage number on product pages or in readily available documentation. When passage number was provided, values ranging from P5 to P15 were observed. Some suppliers describe their process using terms like "young cells" or "early passage" without providing a specific number.

The economic incentive for high-passage production is clear: each additional passage doubles the cell number and therefore doubles the potential exosome yield from a single starting tissue donation. A supplier who harvests at P10 versus P3 produces approximately 128 times more cells from the same starting material (2^7 doublings). This creates direct financial pressure against low-passage harvest, and passage number disclosure creates accountability that non-disclosure avoids.

Passage number at harvest is a minimum disclosure requirement for any MSC exosome preparation represented as reflecting MSC biology. The absence of this information should be treated as disqualifying.

Key References

Continue: MSC Characterization Standards

ISCT criteria explained, why surface markers alone are insufficient, and the full COA requirements for a characterized exosome preparation.

Characterization Standards → Browse Literature Find a Provider