BAL in COVID-19: cellularity, immunophenotype, and cytokine
profile.
BAL characteristics and cellularity can be extremely useful in clinical
practice, helping to identify possible differential diagnosis, and to
guide the diagnostic and therapeutic choice of clinicians. BAL and lung
cryobiopsy represent unique specimens to investigate the excessive
inflammatory pulmonary response to SARS-Cov-2 that represent a major
cause of disease severity and death.44,45 Doglioni et
al elegantly described the histological and immunohistochemical features
observed in the early-phase COVID-19, in cryobiopsies performed in
non-intubated patients, with perivascular CD4-T-cell infiltration,
capillary and venular changes, florid alveolar type II cells
hyperplasia, no hyaline membranes.45 The T-cell
perivascular infiltrate was CD 4 positive, but negative for functional
activation markers (T-BET, FOXP3, CD25 and CD 30). Few interstitial PD1
+ and TCF 1+ T CD8+ lymphocytes were detected. NK cells (CD 56+) and
B-cells (CD 20 +) were rare or absent.45 BAL studies
can provide precious data on the cellular and molecular component from
the distal lung, that nicely integrate histology findings. Compared to
lung biopsy BAL is much more easily performed, therefore a considerable
number of recent studies have used BAL to evaluate the alveolar cellular
profiles that could correlate with clinically meaningful outcomes (e.g.
disease severity and mortality) and that could help the understanding of
COVID-19 pathogenesis. Dentone et al, described the BAL characteristics
and cellularity of 64 COVID-19 patients admitted during March and April
2020 to the Intensive Care Unit (ICU) of Genoa Hospital. 34,4% had
coinfections detected by BAL (Candida, Psedumononas, Enterobacter
aerogens, Staphylococcus aureus and Klebsiella
Pneumoniae).46 BAL samples from individual patients
were taken and their total cellularity, subpopulations, and T
lymphocytes activation as HLA-DR expression.46 The
median cellularity was 68 x 103/ml (IQR 20-145). The
majority cells in BAL were neutrophils (70%, IQR 37.5-90.5), followed
by macrophages (27% IQR 7-49). Eosinophils were less than 1% (IQR
0.9-3). Lymphocytes were a minority, 1%, with CD3+ 92% (IQR 82-95).
Among CD3+ T lymphocytes 52% were CD8+ (IQR 39.5-62.7), with a T
CD4+/CD8+ ratio of 0.6 (IQR 0.4-1.2). 20% where HLA-DR+ (IQR 13-32). At
multivariate analysis only the percentage of macrophages in the BALF at
the time of ICU entry correlated with higher mortality (OR 1.336, 95%
CI 1.014-1.759, p = 0.039). The duration of mechanical ventilation was
correlated with percentage of TCD8+ in BALF (r = − 0.410, p = 0.008),
TCD4+/CD8+ ratio (r = 0.425, p = 0.006) and total lymphocytes TCD3+ (r =
0.359, p = 0.013) in BALF, respectively. The Authors speculate that the
lack of lymphocytes in the BALF in patients admitted to the ICU could
partly explain a reduced antiviral response. The reason for this
depression of lymphocytes could be related to both direct virus damage
to the lymphocyte and by cytokine storm induced
damage.46 That innate immunity is extensively
activated has been confirmed also by Pandolfi et al, that in the BALFs
of 33 adults admitted to the ICU reported a marked increase in
neutrophils (1.24 X 10^5 ml, 0.85-2.07), reduced numbers of
lymphocytes (0.97 X 10^5 ml, 0.024-0.34) and macrophages (0.43 X
10^5 ml, 0.34-1.62) with viral particles inside mononuclear cells
(seen by electron transmission microscopy and
immunostaining).47 The majority of BAL showed
coinfections (26/28). The burden of pro-inflammatory citokines was
associated with clinical outcome, IL-6 and IL-8 were significantly
higher in ICU patients than in Internal Medicine Ward (IL6 p <
0.01, IL8 p < 0.0001), and also in patients who did not
survive (IL6 p < 0.05, IL8 p = 0.05 vs. survivors).47 A recent study by Reynolds and co-workers showed
that inflammatory immune dysregulation of the lower airways during
severe viral pneumonia (both severe influenza and SARS-Cov-2 were
included) is distinct from that of non-severe illness, with an influx of
non-classical monocytes, activated T cells and plasmablasts B cells. BAL
cytokines were elevated in severe cases, but not in moderate patients.
Largest elevation were observed in IL-6, IP-10, MP-1 and
IL-8.48 Contrarily to previous reports, Gelarden et al
reported in 83 patients intubated for severe COVID-19 a lymphocytosis
(i.e. > 15%) in 74.7% of cases (62/83) with a high
prevalence of atypical lymphocytes in BAL (72.3%,
60/83).49 BAL lymphocytes, including plasmacytoid and
plasmablastic cells, were composed predominantly of T cells with a
mixture of CD4+ and CD8+ cells. Both populations had increased
expression of T-cell activation markers, suggesting important roles of
helper and cytotoxic T-cells in the immune response to SARS-Cov-2
infection in the lung. BAL lymphocytosis was significantly associated
with longer hospital stay (p < 0.05) and longer requirement
for mechanical ventilation (p < 0.05), whereas the median
atypical (activated) lymphocyte count was associated with shorter
hospital stay (p < 0.05), shorter time on mechanical
ventilation (p < 0.05) and improved
survival.49 All these data should be interpreted with
great caution because are derived from small, retrospective and
monocentric studies with an evident heterogeneity between cohorts in
terms of phenotypes, disease severity, duration of intubation, presence
of coinfections. Moreover, there is a critical lack of BAL data in
non-intubated patients with less severe COVID-19, that limit our ability
to understand disease pathogenesis in the early phase of the disease.
Besides those evident limits, the current body of evidence suggests that
BAL cellular analysis is an invaluable tool to provide useful
information for diagnostic and prognostic workup and potentially to
expand our understanding of COVID-19 pathogenesis.