Discussion:
In this study we describe cellular immune signatures characterizing mild
(asymptomatic as well as symptomatic) and hypoxemic (moderate and
severe) COVID-19 disease. We further elucidate the role of sMAdCAM, an
important mucosal and lymphocyte migration marker and IL-15, a cytokine
regulating leukocyte activation and homeostasis, in the observed
cellular immune signatures.
Our results clearly illustrate, as has been well described (13–16),
leukopenic profiles only in hypoxemic COVID-19 affected individuals.
Further, in-depth analysis of T cell subsets revealed distinct depletion
profiles in operation within CD4+ and CD8+ T cell compartments. While
the CD4+ T cell compartment did not show any preferential depletion
across naïve and memory subsets, we report here an asymmetric depletion
profile in hypoxemic COVID-19 infection characterized by drastic
depletion of naïve CD8+ T cells with relative sparing of the CD8+
effector memory T cell subset. These results extend the limited data
available for T cell subsets in a stratified manner to include distinct
stages of COVID-19 pathogenesis (10, 17). The CD4+ T regulatory cell
subset represents a critical part of the response to viral infections
(18, 19) that may play both protective as well as exacerbatory roles in
COVID-19 pathogenesis. Our analysis revealed an incremental decrease in
absolute count of this subset from moderate to severe pathogenesis
compared to healthy controls. However, both stages of hypoxemic
infection showed a relative preservation of this subset evidenced
through elevated frequencies compared to healthy controls. As has been
reported very recently this elevated frequency may represent suppressive
(protective) as well as inflammatory roles played by these cells,
markedly higher, in severe disease (20). Our study also evaluated the
non-lymphocyte compartment in terms of monocytes and their profiles in
mild and hypoxemic COVID-19 disease. This compartment has been known to
modulate inflammatory responses to a variety of viral infections (21,
22). Interestingly, as opposed to the T cell compartment and when
compared with healthy seronegative individuals, a pan-disease signature
was delineated within monocytes comprising of elevated frequencies of
intermediate monocytes. Also, concomitant decrease in frequency of
non-classical monocytes was observed in mild infection irrespective of
symptoms.
In an effort to understand factors influencing the observed cellular
depletion and dysfunction in our cohort we evaluated plasma markers that
could influence inflammation, mucosal leukocyte migration and
homeostasis. Extending and strengthening our previous results on the
converse relationship between sMAdCAM and IL-6 observed in non-hypoxemic
COVID-19 progression (12), we report a similar and inversely correlated
relationship between sMAdCAM and IL-6 levels across mild and hypoxemic
stages of COVID-19 disease. Decreased sMAdCAM levels observed following
SARS-CoV-2 infection in our study complement recently reported findings
of reduced circulating lymphocytes expressing integrin
α4β7 in peripheral blood of COVID-19
patients. Furthermore, the restoration of both sMAdCAM levels (in our
study) and α4β7 expressing lymphocytes
(23) was observed in convalescent individuals. Taken together, these
results highlight a clear role for altered mucosal homing in the
pathogenesis of this disease Furthermore, for the first time, we show
that this converse relationship extends to opposing correlations with
observed cellular immune signatures defining COVID-19 pathogenesis.
Notably, elevated circulating LPS (a marker for gut inflammation), in
the context of SARS-CoV-2 infection, has only been reported for severe
and ICU patients (24). In our study we also observed increased plasma
LPS levels in hypoxemic individuals compared to seronegative controls as
opposed to our previous work with a cohort of mild COVID-19 (12).
Importantly, there was no correlation between circulating LPS levels and
either sMAdCAM or IL-6 levels. These results suggest that the altered
trajectory of sMAdCAM levels is less related to gut persistence mediated
inflammation and more to altered mucosal leukocyte migration. Taken
together, our results show that IL-6 and sMAdCAM may contribute towards
disparate aspects of disease progression where circulating levels of the
former are more closely related to inflammatory sequalae and development
of anti-viral B cell responses (25, 26) and those of the latter with
restoration of mucosal homeostasis of leukocytes. Indeed, we have
recently highlighted a role for the modulation of sMAdCAM and expression
of its receptor integrin alpha-4 beta-7 present on monocytes,
lymphocytes in the context of HIV pathogenesis (27).
While most immune markers of pathogenesis in COVID-19 have been reported
to be associated with mild, moderate and severe disease, a marker that
clearly discriminates between symptomatic and asymptomatic pathogenesis
remains elusive. Our study is the first to demonstrate the presence of
plasma IL-15 as such a marker where detection of this cytokine is
clearly segregated based on the symptomatic status of patients with
incrementally higher levels detected in mild symptomatic to hypoxemic
individuals. In fact, of the 2 (out of 11) mild asymptomatic individuals
that had detectable levels of this cytokine, one was found
retrospectively to have progressed to a hypoxemic stage. Also, negative
correlation of plasma IL-15 levels with lymphopenic profiles (T, B, NK)
are indicative of its homeostatic restorative production (28–30) in the
face of systemic depletion of these subsets. Circulating IL-15 levels
have been implicated as a contributory factor to hospitalization time,
disease severity and mortality in some settings (31, 32). Indeed, a
preliminary analysis in our cohort, with respect to days of
hospitalization supported this finding. Our study further demonstrates
the utility of using it as an early biomarker predicting progression to
symptomatic and severe disease.