Endothelial inflammation: how many bad apples?

In this issue of Blood, Caruso and colleagues¹ provide compelling evidence that when the exceptional flexibility of erythrocytes is impaired, the resultant aberrant wall interactions during flow induce a molecular signal of endothelial inflammation that extends the classic model of vascular dysfunction in sickle cell disease (SCD).

Interfacial observations, such as those between organ systems and indeed various disciplines, have resulted in outstanding advancements in biomedical science. Early optical observations of the microcirculation, for example, highlighted the presence of a cell-poor region² between blood and the vessel wall that spurred a generation of biophysical and rheological inquiry. Evolution of studies exploring cellular mechanotransduction, where a physical stimulus is converted into a molecular response, led to Nobel Prizes in Physiology or Medicine for unraveling the vital processes underlying the flow-induced synthesis of species involved in vasoregulation (1998) and the transport of cations across cell membranes via mechanosensitive channels (2021). In contrast, advancements in the understanding of pathophysiology and formulation of effective therapies are often frustratingly incremental. For instance, although the characteristic impairment of erythrocyte deformability in SCD has been rigorously linked to a wide range of complications and comorbidities, the precise mechanistic nature of these relationships remains elusive, leaving important questions unsatisfactorily resolved.

A considerable challenge in addressing whether erythrocytes are involved in vascular dysfunction arises from determining whether a critical threshold exists, at which compromised erythrocytes influence whole blood rheology, and consequently whether altered vessel wall conditions might exert vascular maladaptation. Further complicating matters is the innate heterogeneous nature of blood, given the physical properties³ and biochemical potential⁴ of erythrocytes vary considerably even within healthy blood samples apparently due to in vivo cellular aging. Since such heterogeneity exists, and even though in hemorheological diseases subpopulations of normal cells with superior properties are present, effort has been directed toward answering the question: how many bad apples does it take to spoil the barrel? We recently demonstrated that when only 1% to 5% of the total erythrocyte population are experimentally rigidified, vital properties of whole blood are significantly altered, particularly when examined under shear rates reflective of the arterial network.⁵ Whether such suboptimal erythrocyte subpopulations could induce clinically meaningful alterations to the vasculature and thus influence organ health, however, remained a considerable evidence gap.⁶ Caruso and colleagues reduce this gap by demonstrating that when the cell-poor region is challenged by rigid subpopulations of erythrocytes, the resultant abnormal interactions with the vessel wall induce proinflammatory signals in endothelial cells. Understanding the processes that govern the cell-poor region are thus fundamental to appreciating this observation.

Under normal laminar flow conditions, erythrocytes are not evenly distributed within blood vessels but rather cluster toward the central region of flow. This “axial migration” of erythrocytes evidently results in the hematocrit, and indeed viscosity of blood, to vary radially across the blood vessel, with the lowest values for each exhibited nearest the vessel wall. The physical explanations for processes governing this observation are fascinating and described within the Fåhræus effect² and the Fåhræus-Lindqvist effect,⁷ respectively. Although the details are beyond the scope of this commentary (although have been clinically contextualized elsewhere⁸), the outcome of this process is that while erythrocytes migrate toward the central axis of flow, platelets and leukocytes marginate in the opposite direction toward the vessel wall. The resultant viscosity of the cell-poor region is thus largely dictated by the fluid component (plasma) rather than by the more viscous erythrocytes. Collectively, under normal conditions these processes contribute to optimized hemostasis, immune function, and nutrient delivery, and the frictional resistance to flow is significantly reduced owing to a lubricantlike effect of the plasma rich, and cell poor, region nearest the vessel wall.

The observations, therefore, by Caruso and colleagues in this issue are not trivial; disruptions to the cell-poor region have been suspected to exert broad impacts to health, although mechanistic evidence remained elusive. They report that when as few as 5% of total erythrocytes were chemically rigidified, or from human donors with SCD, and then perfused through endothelialized models of venules, an inflammatory response was stimulated in endothelial cells as evidenced by aberrant gene expression at the single-cell resolution. Of note, significant increases were observed in robust upstream mediators of endothelial activation, inflammation, and oxidative stress and also leukocyte recruitment, adhesion, and transendothelial migration. In this well-reported era of poor reproducibility, the cross-validation approach employed by the authors is particularly elegant: the physical processes predicted from a rheological perspective are evident in both in vitro and in silico models, improving confidence in the outcomes.

The work of Caruso and colleagues will likely contribute to a deeper understanding of the pathophysiological processes in other disorders characterized by concomitant impairments in vascular function and the rheological properties of subpopulations of erythrocytes. The interpretation of such pathophysiology, especially in the complex context of SCD, has been plagued by a chicken-and-egg dilemma. However, promising studies on the efficacy of exchange transfusion for the improvement of vascular function⁹ and the efficacy of hemoglobin polymerization inhibitors in ameliorating cerebral flow and oxygenation¹⁰ support further innovative approaches addressing the “bad apples” and may catalyze a paradigm shift in clinical outcomes.

Conflict-of-interest disclosure: M.J.S. declares no competing financial interests.