Discussion
The body immune system plays an important role in sepsis like
double-edged sword, which protects against invading pathogens, while its
overreaction can cause body organ injury. During sepsis, the invasion of
a large number of pathogens not only causes the body’s immune
overreaction, but also causes a large number of lymphocytes to consume
and cause immune paralysis. As Hotchkiss RS described that the T cells
were the band conductor of the orchestra, and all immune components
worked in harmony to play the best role[30]. Due to the intertwined
complex relationship, Th17, Treg and their balance proportion has been
attracted more and more attention, which is essential for immune
homeostasis[31]. Although Treg represent less than 10% of
circulating CD4+T cells, it can limit the overreaction of anti-infective
effector cells, protect surrounding normal tissues from damage, and
maintain the body’s immune balance [32, 33]. Th17 terminally
differentiated cells fulfill opposite Treg’s functions; IL-17 can
promote the occurrence of inflammation in sepsis[34]. Studies have
found that Th17 can appear in lung tissue to help eliminate bacteria in
the bacterial infection model, including Klebsiella pneumoniae that is
resistant to carbapenem[35], which is very important to resistance
in sepsis bacterial infection. Th17 cause autoimmunity and inflammation,
whereas Treg inhibit these phenomena and maintain immune homeostasis.
The two are functionally antagonistic to each other, but also inhibit
each other in differentiation, even switch between each other[36].
Thus, unraveling the mechanisms that affect the Th17/Treg balance is
critical to understand autoimmunity and tolerance better. Limited data
had shown the ratio of Th17/Treg might be related to the clinical
severity and prognosis of sepsis [37], [38], [39].
The length of stay in intensive care units (LSICU) is an essential
factor for quality assessment and a strong determinant for the total
cost of ICU admission, of which septic patients represent a major
part[40]. Age was an independent risk factor of many diseases, and
some researches showed that the odds for death in ICU patients with
sepsis increased with age with the maximal rate of increase [46].
Age is a factor affecting the length of stay in ICU in patients with
sepsis, and there is a positive correlation between age and length of
stay in ICU. Clinical scores except age, such as the APACHE II[41],
SOFA[42] have been widely used in clinical practice to predict
outcome in critically ill patients; the early risk-stratification of
these patients and their prognosis, as well as accurate monitoring of
clinical treatment effects. Th17/Treg ratio can predict 28-day mortality
in sepsis patients with ARDS [43]. There is currently an apparent
lack of an accurate biomarker for predicting sepsis patients the length
of stay in ICU. In the present study, positive correlations were
observed between the Th17/Treg ratio and the length of stay in ICU,
indicating that the higher the Th17/Treg ratio, the longer the length of
stay in ICU in sepsis patients. The Th17/Treg ratio, APACHE II score,
and SOFA score were independent predictors of the length of stay in ICU
in sepsis patients, the Th17/Treg ratio in combination with the APACHE
II score increased the AUC for predicting the length of stay in ICU.
Taken together, our findings strongly suggest that the Th17/Treg ratio
can reflect the intensity of the inflammatory response in sepsis, and
might be a potential indicator for the length of stay in ICU in sepsis
patients.
CD28 is the primary costimulatory molecule for naive CD4+ conventional T
cell activation[44], binding to B7 ligands leads to increased
duration and magnitude of T cell responses, enhanced survival and
glucose metabolism, and acquisition of migratory properties[45].
CD28 activates integrin-mediated adhesion of T cells and promotes actin
polymerization. Selective blockade of CD28-mediated T cell costimulation
protects rhesus monkeys against acute fatal experimental autoimmune
encephalomyelitis(EAE)[46]. In septic patients, there is a strong
positive correlation between CD28 expression on CD4+ T cells and
Th17/Treg ratio; we speculate that CD28 may be able to change Th17/Treg
ratio then short the length of stay in ICU.
The mice sepsis model showed that the clinical score and Th17/Treg ratio
of septic mice were the highest on the third day after CLP. After
administration of CD28 antibody, the 7-day survival rate of mice
increased, the Th17/Treg ratio of spleen cells decreased, and the
expression of inflammatory factors in peripheral serum also decreased.
Simultaneously, the spleen’s length, weight and index were improved, and
the histopathological damage were alleviated. These suggested the
decrease of apoptosis of spleen cells in mice. It is speculated that
CD28 antibody may affect the balance between Th17 and Treg through
ROR-γt and FoxP3, thereby improving the survival rate of sepsis mice.
The function of T cells is related to cell death, which can be divided
into accidental cell death (ACD) and regulatory cell death (RCD). The
signal cascade reactions involved in regulating cell-related functions
are mainly RCD, RCD involving effector molecules, which have unique
biochemical characteristics, morphological characteristics and
immunological consequences [47]. RCD mode includes apoptosis,
necroptosis, pyroptosis, ferroptosis, entotic cell death, and so
on.[48]. T cell dysfunction is manifested specifically in increased
T cell apoptosis, decreased proliferative capacity and decreased
reactivity or unresponsive state[49]. Our data showed that the
administration of CD28 antibody to CLP processed mouse could reduce the
apoptosis of splenocytes. Western blot in tissues confirmed that the
expression of pro-apoptotic protein BCL-2 decreased and the expression
of anti-apoptotic protein BAX increased in mouse spleen under the action
of CD28 antibody.