Discussion
To our knowledge, this is the first preliminary study evaluating the
lymphocyte subset characteristics of children with laboratory-confirmed
SARS-CoV-2 infection.
Although early studies reported that children were less likely to
develop COVID-19 than other age groups, recent studies revealed that
children are at a similar risk of infection to the general population
(Bi, Wu et al. 2020,
Lotfi and Rezaei 2020). Consistent with
previous reports (Dong, Mo et al. 2020,
Guan, Ni et al. 2020,
Sun, Li et al. 2020,
Tang, Wu et al. 2020), the present study
showed a male predominance in the incidence of COVID-19.
Changes in the immune system may result in decreased immune function and
increased incidence of severity, morbidity, and mortality of infections
(1, 2). According to the laboratory findings, normal leukocytes counts
were more common in both cases with severe and mild disease that was
similar to the previous report in children
(Bai, Liu et al. 2020). It has been
hypothesized that the repletion of lymphocytes plays a vital role in the
recovery of patients with COVID-19 (Henry
2020). Lymphopenia and inflammatory cytokine storm during highly
pathogenic coronavirus infections, such as SARS coronavirus (SARS-CoV),
MERS coronavirus (MERS-CoV), and SARS COV-2 infections might be
associated with disease severities (Yang,
Li et al. 2020).
Although 63%−84.6% of adult patients with severe diseases have
lymphopenia (Guo, Cao et al. 2020,
Liu, Li et al. 2020), in our study,
lymphopenia was found in 19 children (34.5%). Lymphocyte count was
significantly lower in patients with severe disease. So, a significant
decrease in lymphocytes may signify clinical worsening and increased
risk of a poor outcome. According to previous reports in children with
probable or suspected SARS (Bitnun, Allen
et al. 2003, Hon, Leung et al. 2003),
total lymphopenia was common and more prominent in older children with
more severe disease. The counts of total leukocytes, RBCs, platelets,
and neutrophils were not significantly different in severe and mild
groups.
Inflammatory markers, including CRP and ESR, were higher in severe cases
than in mild cases, demonstrating a higher inflammatory state during
severe infection. Since cytokine storm is associated with apoptosis of
lymphocytes, a decrease in number of peripheral lymphocytes in severe
cases is probable (Ni, Tian et al. 2020).
Although an apparent decrease in peripheral lymphocytes in COVID-19
patients has been reported, alteration in the T cells subsets is still
unclear (Wang, Nie et al. 2020,
Zhang, Zhao et al. 2020). However, it has
been reported that CD4+ T cells and
CD8+ T cells decreased in severe cases of COVID-19
than mild cases (Liu, Li et al. 2020,
Ni, Tian et al. 2020,
Wang, Nie et al. 2020). In our study
higher number of CD8+ T cells was documented in the
severe group than mild cases. The CD4+ lymphocytopenia
was more prevalent in the severe group than the mild group (38% vs.
29%). On the other hand, higher percentage of CD8+ T
cells was found in the severe group (29%) than in the mild group
(18%); however, these differences were not significant. In the study of
Bai et al. (Bai, Liu et al. 2020),
Among 25 asymptomatic cases, very mild cases, 23 cases had normal
CD8+ T lymphocyte counts, while increased
CD8+ T lymphocyte counts were mildly increased in two
cases (8%).
Progressive loss of CD4+ T cells and an increase in
CD8+ T cells are consistent features of HIV-1
infection. It has been reported that CD8+ T cells’
count may change slightly, while the CD4+ T cells
count decline at inconstant rate. A progressive decline in
CD4+ T cells, as well as a rise in
CD8+ T cells, suggests a compensatory rise in
CD8+ T cells in response to a decline in
CD4+ T cells (Margolick,
Munoz et al. 1995).
Although lymphocyte count, especially CD4+, has been
reported as a clinical predictor of severity of disease
(Henry, de Oliveira et al. 2020), in our
study, no significant differences between CD4+ T cells
and CD8+ T cells and severity of disease were found.
On the other hand, the CD4+/CD8+ T
cell ratio was significantly lower in those patients with severe disease
compared to those with mild disease. In contrast to previous reports in
adults that absolute numbers of total T lymphocytes,
CD4+ T cells, and CD8+ T cells were
reduced below the lower limit of normal in the vast majority of patients
with either severe or moderate COVID-19
(Wang, Hu et al. 2020), we demonstrated
that percentage of CD4+ T cells, and
CD8+ T cells were reduced below the lower limit of
normal in 29% and 38% of cases with mild and severe COVID-19,
respectively. However, CD8+ T cells were reduced below
the lower limit of normal in 12% and 5% of cases with mild and severe
COVID-19, respectively.
The low frequency of lymphopenia in severe patients is mainly related to
the significantly decreased absolute counts of T cells, especially
CD8+ T cells (Liu, Li et
al. 2020), therefore; low rate of lymphopenia in children might be due
to the fact that a majority of children have normal or even elevated
CD8+ T, cell counts.
The normal CD4+/CD8+ ratio in
healthy hosts is poorly defined. Ratios between 1.5 and 2.5 are
generally considered normal (McBride and
Striker 2017). Low CD4+/CD8+ ratios
mainly observe in HIV. However, a low or inverted
CD4+/CD8+ ratio is associated with
systemic lupus erythematosus, chronic inflammation, and cytomegalovirus
infection as well (Maeda 1999,
McBride and Striker 2017).
CD8+ T cells represent an important immunological
component of protection against viral infections. Cytomegalovirus has a
significant impact on the CD4+/CD8+ratio by increasing circulating CD8+ cells in response
to the infection (Reker-Hadrup, Strindhall
et al. 2006).
Inversion of the CD4+/CD8+ ratios
are associated with poor survival
(Huppert, Pinto et al. 2003), a link
between low ratio and poor outcomes observed in our study, and
importantly 86% of death rates (n=6) was reported in cases with low
CD4+/CD8+ ratios (<1.5).
A decrease in CD4+/CD8+ ratio
indicates the extent of the impairment of the immune system by COVID-19.
Therefore, a decline in CD4+/CD8+ 8
ratio may serve as a useful factor to reflect the derangement of immune
responses and even death in patients with severe COVID-19.
In conclusion, lymphocyte counts and inverted
CD4+/CD8+ T cell ratio can be
considered as a potential predictor for disease severity of COVID-19.