Atypical Monocytes in COVID – 19: Igniting the Fire of the Cytokine Storm? – Pence – – Journal of Leukocyte Biology

The emergence of severe acute respiratory syndrome coronavirus – 2 (SARS – CoV – 2) and its subsequent coronavirus pandemic disease 2019 (COVID – 19) has accelerated a huge research effort to characterize the immune system’s responses to the virus. Given the public health and economic burdens of the pandemic, much of this research has focused on efforts to produce immunogenic vaccines and develop effective antiviral treatments against SARS – CoV – 2. Early in the pandemic, however, it was observed that a severe severe COVID-19-associated inflammatory cytokine storm,¹ and as such, some effort has been made to characterize the etiology of this phenomenon.

Given their known leading role in mediating early inflammatory responses in the innate immune system, monocytes and macrophages have naturally received considerable attention. In the early months of the pandemic, a series of observational studies were published that noted alterations in the populations and functions of circulating monocytes during COVID-19 of varying severity. These changes have been somewhat inconsistent in the literature, but the overall pattern includes population increases in pro-inflammatory and disease-associated phenotypes such as intermediate and non-classical monocytes, as well as downregulation of monocyte HLA-DR expression and increased expression of inflammatory pro-cytokines such as interleukin (IL) -6. For a more complete discussion of these early observations than is possible here, I point readers to my previous review on this topic,² which reviewed the literature on monocytes and macrophages relevant to COVID-19 until about mid-June 2020. This editorial will focus primarily on the most recent findings, including some limited empirical studies, relevant to monocyte biology and COVID-19 in context of the article.

In this issue of Journal of Leukocyte Biology, Zhang et al.³ report an observation of an unusual circulating population of monocytes with pronounced vacuolation under the microscope and increased forward scatter by flow cytometry, in patients with COVID-19. These monocytes exhibited high expression of the non-classical monocyte marker CD16, as well as an increased expression of the macrophage markers M1 and M2, including CD80 and CD206, respectively. Furthermore, monocytes from COVID – 19 patients had enhanced intracellular staining for the pro-inflammatory cytokines tumor necrosis factor α and IL – 6 and the anti – inflammatory cytokine IL – 10, providing further evidence for a heterogeneous monocyte response including aspects of both type 1 and type 2 innate immune responses.

In my opinion, the most important observation from this study is likely to be the altered distribution of monocyte subpopulations during COVID – 19. Human monocytes are most often grouped based on the relative expression of CD14 and CD16 in classics (CD14⁺CD16^–), intermediate (CD14⁺CD16⁺) and non-classical (CD14^BassCD16⁺) phenotypes.⁴ Monocytes expressing higher levels of CD16 (i.e., from intermediate and non-classical phenotypes) are often associated with greater disease severity. However, while this is a routine biomarker or predictor of disease severity in immunological research, it is not well understood whether these cells directly contribute to disease or are simply a byproduct (and therefore a convenient indicator) of severe disease.

Indeed, this is a still unanswered question from COVID-19 observational studies such as Zhang et al. and other studies on the distribution of monocytes during this disease. The extent of the phenotypic changes reported by Zhang et al. it is not known to occur in individuals with chronic disease or pro-inflammatory conditions, and therefore SARS-CoV-2 infection likely has an impact on circulating monocyte populations. However, pre-existing differences in monocyte subpopulation distribution may also play a role in determining disease severity in COVID-19 patients. Older age and obesity are perhaps the two biggest risk factors for severe COVID-19 and CD16⁺ Monocyte subpopulations are known to increase with aging⁴ and obesity.⁵ Although it is not yet fully understood whether basal inflammation contributes to COVID-19 severity, the association between COVID-19 severity and CD16⁺ monocytes, as well as the link between these cells and risk factors such as age and obesity, suggest that altered monocyte subpopulations may be an etiological factor of COVID-19 disease severity regardless of the direct impact of infection on circulating proportions of these cells. Although there have been some high-profile attempts to characterize the kinetics of the immune system during COVID-19 which have shown disease-related expansion of CD16⁺ monocytes,⁶ the lack of background data before infection in these studies renders this question currently unanswered.

Furthermore, while CD16⁺ increases in monocytes during COVID-19 are supported by early observations from other groups (reviewed in Ref. 2), recent high-profile studies have also found decreases in these cell populations as COVID-19 severity increases.⁷ The regulation of the monocyte phenotype by SARS – CoV – 2 is therefore probably multifactorial and the contribution of various subpopulations to COVID – 19 may depend on immune or other responses not yet understood.

Zhang et al. They also provide confirmatory (but nonetheless important) information essential to postulate a role for monocytes in the COVID-19-associated cytokine storm. That is, they provided data demonstrating viremia in COVID-19 patients, as well as expression of the virus receptor (ACE2) on monocytes. While others have described these as well, this remains important as (a) the virus must be in the blood to meet the circulating monocytes and (b) those monocytes must express a receptor for the virus in order to activate an inflammatory response. An increase in monocyte-derived macrophages in the lungs during COVID-19 has been described,⁸ thus monocytes certainly have an important role to play as precursors to infiltrated macrophages in the lungs, but the role of undifferentiated monocytes in contributing to the cytokine storm during COVID-19 remains controversial. However, the observations that monocytes express the cellular receptor for SARS – CoV – 2 and that SARS – CoV – 2 is detectable in the blood are key preliminary indicators that these cells may be important contributors to the systemic pathology of COVID – 19. Indeed, studies in vitro confirmed that SARS – CoV – 2 infection induces cytokine release and metabolic activation in isolated human monocytes,⁹ giving more credit to the role of monocytes in mediating the cytokine storm. However, although these are important observations, it remains to be determined whether the findings in this paper reflect direct stimulation of monocytes by SARS – CoV – 2 or whether these changes are indirect resulting from a change in the monocyte environment. Figure 1 shows a summary of the results of Zhang et al. And some important remaining questions.

In summary, Zhang et al. provided important data reflecting the potential contributions of monocytes to the viral cytokine storm during COVID – 19, as well as observations reflecting a possible role of changes in monocyte subpopulations in mediating disease severity. While important questions remain and empirical evidence for these roles for monocytes is currently lacking, this paper adds to a growing body of literature reflecting the role of dysregulation in the myeloid cell compartment as a basis for COVID-19 severity.