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Fibrosis: A Serious Unmet Medical Need

Fibrosis—commonly known as scarring—can exact a heavy toll on sufferers. In the developed world, a surprising 45% of all mortality is driven by severe fibroproliferative disease processes of the inner organs and circulatory system. [1, 2, 3] These include common atherosclerosis, but also idiopathic pulmonary fibrosis (IPF), liver and kidney fibrosis, systemic sclerosis, and abdominal adhesions. Approved medications to treat fibrosis directly remain of limited efficacy, only relieving symptoms. With the exceptions of nintedanib and pirfenidone for IPF, [4] these drugs usually don’t directly antagonize the molecular drivers of this disease class as we currently understand it.

Figure 1A

Figure 1B

Above, Figure 1. Despite contributing to 45% of all deaths, fibrosis remains an unmet and neglected medical need. Figure 1A, the fraction of annual PubMed hits that queried to terms “cancer” (blue bars) and “inflammation” (orange bars) increased across three decades, while remaining stable for the query term, “fibrosis” (green bars).  Figure 1B, the number of NIH grant awards that query to the terms “cancer” (blue bars) and “inflammation” (orange bars) increased across three decades, while overshadowing the number of grant awards that query to the term “fibrosis” (green bars).

Approximately 8% of all PubMed publications query to the term “fibrosis,” and this estimate has remained flat for three decades. Despite this, long-term trends demonstrate remarkable increases in journal activity for similar queries to PubMed via “cancer” or “inflammation” terms (Figure 1A).  These are, of course, alternate disease classes that are presently targeted by a diverse armamentarium of experimental and marketed drugs. Mirroring annual cancer and inflammation-related publications are commensurate increases in the number of NIH-funded projects (Figure 1B). At long last, intensive research would seem to be starting to turn the tide against cancer after many years (Figure 2). According to the United States CDC, the annual percent of cancer’s mortality decreased from a high of 23% of all 1999 deaths (200.8 per 100K persons), down to below 20% by 2020 (144.1 per 100K). Nevertheless, queries to the NIH RePORTER web portal would suggest that the number of fibrosis-related funded projects reaches a level of only about 10% of cancer grant activity. The COVID-19 pandemic and lingering “long COVID” has particularly heightened recent interest in lung fibrosis, [5] which may continue to affect as many as 44.9% of those who endured serious infections. [6] Therefore, fibrosis remains a serious unmet medical need. [7]

Figure 2

Above, Prior to 2020, USA all-cause mortality (blue bars) was slowly inching down. Of this, the fraction of deaths due to cancer (orange hatched line) also experienced a decline in the 2010s.

We Know Enough to Learn Much More

Both cancer and fibrosis share a resemblance to a wound-healing process gone awry. Sometimes, cancer and fibrosis co-conspire for tumor stromal barriers, pathological angiogenesis, and immune privilege in the same anatomical “crime scenes.” [8, 9] However, malignancy distinctly tends toward cell growth and genomic instability, with natural selection of variant clones for metastasis beyond the unhealed “wound.” In contrast, pathologic fibrosis stays put. That is, a fibrotic disease is the non-homeostatic release of excess, stiff extracellular matrix that precludes the normal restoration of a damaged and/or infected tissue.

The difference between the tendency for quick and regenerative wound healing in young children versus older adults might be evolutionarily hard-wired. [3] In the wild, deep wounds are sites of infection that could be cause for almost instant death from sepsis. With more energy/caloric resources invested into care of “tenderfoots” as future generations of mammals, the ancient tradeoff may alternately tip in favor of “quick and just good enough” scar formation as adults attain breeding and combat age. Here, the older siblings and parents could somewhat benefit from a tough scar’s prevention of excess blood loss and barrier function against invading pathogens, or repeated insults to the old injury site. (Just ask any guitarist, fiddler, or bassist; rough, calloused skin on fingertips isn’t so bad!)

The blurred ontology between infection response, inflammation, and fibrosis is supported by a massive literature on the interplay of immune cell types (neutrophils, macrophages, B cells, T cells), epithelial cells, and mesenchymal cells like fibroblasts. [7, 10, 11] Natural wound healing involves a choreography of these players’ entrances and exits from the lesion, along with characteristic changes in cytokine and chemokine milieu. In the young adult, scarring may gradually resolve over time as the tissue remodels. But in the elderly, this fibrosis might be closely tied to aging itself, perhaps due to “epigenetic noise” where cells gradually lose their identity and/or responsiveness to neighboring cues at the level of randomly silenced chromatin. [12, 13] The enduring presence of damage-signaling molecular patterns in a fibrotic lesion—populated by post-replicative, senescent cells addled by epigenetic nose—may elicit a low-level autoimmunity and/or what has been popularly called “inflammaging.” [11, 14, 15] Such local inflammation may in fact feed back into the pathology, with some T cell subsets (e.g., Th17) self-perpetuating this fibrotic lesion [10, 16] together with fibroblasts [17] in a specific context that can be heavily influenced by TGF-b/Smad signaling, along with IL-4, IL-6, IL-13 and other mediators. [3]

TGF-b recruits fibroblasts that, in turn, terminally differentiate into myofibroblasts, which are contractile cells of intermediate morphology between smooth muscle cells and fibroblasts. [18, 19] With lesions healing nominally, these die by apoptosis while the wound is resolved—gradually replaced by the familiar residents of soft tissue architecture. Yet in hypertrophic fibrosis, myofibroblasts may continue to perpetuate a TGF-b autocrine loop that suppresses proximal epithelial cell growth and induces endothelial- and epithelial to mesenchymal (EMT) transitions, [20] increasing the margins of the scarring. Obviously, fibrotic disease is more complex than briefly summarized here and context-dependent—varying between tissues and early to late phases of disease. [21] Along with highly context-dependent fibrosis presentation, there comes an inevitable challenge to treat the condition due to narrow windows of efficacy via molecular targeted drugs. [4, 9, 21] This includes pharmacopoeia to antagonize TGFb, where total systemic blockade is not without major toxicity. [9, 21]

MSCs & Their EVs/Exosomes – Thinking Globally, Acting Locally

Injected mesenchymal stromal/stem cells (MSCs) reportedly home toward in vivo toward injuries under certain experimental conditions, and they also may do so endogenously. [22, 23, 24] When implanted locally at the injury site—or artificially encapsulated in proximity—their secreted factors and vesicles are widely observed to contribute to rapid tissue regeneration in animal studies. [25, 26, 27] Consistent with MSCs’ original role as pericytes that activate near punctured blood vessels and support their repair, [27, 28, 29] MSCs are the basis for over 1500 worldwide clinical trials posted to international databases since 2011. [30] Their overall track record in human patients has been quite safe, and hence they comprise the active cellular material for approximately 10 approved therapies beyond the United States. [31] MSCs’ capability for artificial targeting, in situ, via topical paracrine secretome activity may open new avenues for combatting fibrosis that might otherwise be too risky for systemic small molecules or mAbs.

Potent anti-inflammatory, pro-angiogenic, anti-microbial, and anti-fibrotic features of MSCs are widely reported. However, MSCs also are a heterogeneous mix—sourced from different tissues via different isolation methods, from different donors who each exhibit different ages and health states. Today, the mechanisms of action of MSCs are strongly believed to be related to their secretory profiles. Still, the reductionist quest to isolate their most essential factors remains perplexingly elusive. [32] A confounding feature is that MSCs may also, themselves, differentiate into myofibroblasts and/or perpetuate fibrosis, being responsive to TGF-b. [33] Therefore, as a double-edged sword, it becomes imperative to fastidiously characterize MSCs according to ISCT guidelines [34, 35] and transparently publish all details of their sourcing, isolation, expansion, dosing, and administration methodology so that the state-of-the-art can advance along well-defined quality attributes (CQAs). [36]

Multiple differentiation states of MSCs may seem at first glance as an obstacle to future clinical translation. A steadily increasing volume of published research suggests this might be surmounted, however (Figure 3). MSCs are readily engineerable cell systems, so their broad profiles of regenerative “features” are not “bugs” at all. Our own group at RoosterBio—and many others—have demonstrated that the therapeutic phenotype of MSCs can be dramatically altered through priming pretreatments, [37, 38] high-performance media, [39] and genetic modification. [40] Among other means, heterologous overexpression of the putative anti-fibrotic hepatocyte growth factor (HGF) in MSCs might be particularly effective. [41] In addition, MSCs are prolific producers of extracellular vesicles (MSC-EVs), including exosomes. [42] cGMP-bioprocessed exosomes can be collected from “domesticated” MSC producer cell supernatants [43, 44, 45] that are analogous to the “workhorse” CHO cells for mAbs or HEK-293Ts that yield clinical lentivectors. These lipid-bound particles are themselves able to ameliorate ex vivo and in vivo disease models of fibrosis. [46] Likewise, MSC-EVs are customizable by genetic modification of the host cell and/or by artificial modifications that are post-harvest. [47] The surfaces of MSC-EVs could express multivalent, tissue-selective binding and internalization moieties (including scFvs), [48, 49] and their contents can be enriched with small molecule, protein, miRNA, siRNA, mRNA, or (epi)genome editor cargoes to radically alter the course of fibrotic or other modeled diseases. [50] Thus—by steering away from off-tissue effects as a designer drug delivery platform—both MSCs and MSC-EVs can attack multiple and simultaneous fibrotic molecular targets once dismissed as “undruggable.”  [51]

Figure 3

Above, PubMed search queries may indicate increasing research interest in use of MSCs (blue bars) or MSC-EVs/exosomes (orange bars) for novel anti-fibrotic therapies.

“Think globally, act locally,” exclaims many a bumper sticker. Like most tailpipe memes, it sounds nice enough—but this bromide is obviously difficult to pin down for practical specifics. Yet, one would be stumped to think of a more geographically dispersed cause of historical human misery than organ fibrosis. And one would be hard-pressed to imagine anything more local than a therapy within the human body, acting as a “magic bullet” with molecular and spatiotemporal specificity against a disease process. Too good to be true? Maybe not! With MSCs and their EVs, have we finally approached the ultimate example of a cause worth fighting for?

 

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