Introduction
Autoimmune neutropenia (AIN) in children is characterized by a low
absolute neutrophil count caused by the excessive destruction of
neutrophils through antibodies against human neutrophil antigens
(anti-HNA Abs) (1). The median patient age at diagnosis of AIN is 7–9
months (2, 3). During the period of neutropenia, many patients tend to
present bacterial, but not severe infections, because their bone marrow
is intact and immediately produces neutrophils after receiving an
infectious signal. Some patients receive prophylactic medication, such
as sulfamethoxazole-trimethoprim combination to avoid recurrent
infections (4). Unlike other autoimmune diseases with autoantibodies,
AIN is a self-limited disease which does not require any special
therapies (e.g., steroid medications). Because anti-HNA Abs disappear
gradually in many cases, almost all patients recover without treatment
in 2–3 years (3, 5).
Although AIN is not a rare neutropenia of childhood, its
etiopathogenesis remains unclear. Several researchers have attributed
causes of this disease to the modification of antigens after exposure to
drugs, molecular mimicry of microbial antigens, post-infectious
autoantibodies, and differences in human leukocyte antigen types (5–8).
Previously, we reported a deficiency of regulatory T cells (Tregs;
CD4+CD25+ FOXP3+ T
cells) in children with AIN as another cause of this disease (9). Tregs
play a key role in suppressing the immune response based on the control
of autoimmunity in peripheral tissue (10). In fact, the deficiency of
Tregs has been shown in several autoimmune diseases (11). Furthermore,
Tregs could separate subpopulations such as resting Tregs, activated
Tregs, and non-suppressive Tregs according to the extent of expression
of CD45RA and FOXP3. Activated Tregs, defined as
CD4+CD25+FOXPhighCD45RA−T cells, have the most suppressive function in these subpopulations
(12), and the fluctuation of the aforementioned three subpopulations is
associated with autoimmune diseases (13, 14).
T cells, in combination with B cells, play an important role in antibody
production. For the recognition of various antigens, each T cell has a
specific T cell receptor (TCR) on its surface (a heterodimer comprised
of α- and β-chains). TCR is formed by random re-combinations of TCR gene
elements termed V, D, J-segments (15). The random re-combination causes
diversity of the TCR repertoire. The complementarity determining region
on the β-chain made from the V-region is the most important region for
diversity (16). Recently, associations with
repertoire of TCR-Vβ and
autoimmune diseases, such as systemic lupus erythematosus (SLE), type 1
diabetes mellitus, autoimmune thyroiditis, and idiopathic
thrombocytopenic purpura, have been reported in numerous research
studies (17–20). The results have shown several different usages or
expansions of the TCR-Vβ family in each disease and indicated an
association between disease and an unusual repertoire of the TCR-Vβ
family. However, there has been no study analyzing the repertoire of the
TCR-Vβ family in patients with AIN.
Although the high throughput
sequencing is often used for TCR-Vβ repertoire analysis nowadays, it
requires much cost. The analysis using flow cytometry could give us less
information than high throughput sequencing, but this method is easy and
useful to get overview of the repertoire of the TCR-Vβ family in each T
cell subset.
In this study, we analyzed the frequency of total Tregs and activated
Tregs in CD4+ T cells. Subsequently, we investigated
the repertoire of the TCR-Vβ family in patients with AIN using flow
cytometry to obtain information on the usages of the TCR-Vβ family in T
cells.