1 – Inborn errors of immunity
Immunology recap
Primary lymphoid organs consist of the thymus and bone marrow. Secondary lymphoid organs are the
spleen and lymph nodes (including the galt, which is a lymph node associated with the gut). The
production of T-cells occurs in the bone marrow, but they are educated in the thymus. Most of the
T-cells that recognize the self-peptides get eradicated but there are T-cells escaping this negative
selection. Not all self peptides can be presented in the thymus thus, this can lead to autoimmunity.
Specialized lymphocytes are part of the innate immune system. They consist of the NK cells, innate
lymphoid cells (ILCs), and gamma delta T cells which are considered to be in between innate and
adaptive. They have TCRs but then have a limited recombination that they only can recognize very
general patterns.
Currently, it is debated that there could be modifications in the DNA (epigenetic modifications) which
makes macrophages more active upon encountering new pathogens (signs of memory in innate
immunity). There are specific cytokines (like IL-17 for Th-17) but most of them are non-specific ones
(TNF-a) which have a lot of different functions. Hypersomatic mutation occurs in the B-cell
development. It is a genetic mutation of the B-cell receptors upon antigen exposure. B cell receptors can
change during the lifespan but T cells cannot change. The isotype is the subclass of antibody while the
idiotype is another name of the variable part that is binding the antigen.
There are different kinds of antibodies. IgG is the most prominent in the serum, it is the one that can
cross the placenta. IgA is predominantly found in the mucosal tissues. IgM and IgG activate the
complement system. IgE activates mast cells and basophils (allergies).
Inborn errors of immunity (IEIs)
There are 3 main functions of the immune system: to fight infections, fight several non infectious
diseases (mainly cancer), and prevent unnecessary inflammation (autoimmunity). When the immune
system fails, it may lead to inborn errors of immunity (IEIs) or previously known as primary
immunodeficiencies (PIDs), usually genetically caused. This deficiency can occur in all parts of the
immune system, including the T and B-cells.
1. B-cells
B-cell deficiency diseases cause low immunoglobulins as no antibodies can be produced. This mainly
leads to bacterial infections in the airways such as sinusitis, ear infection, bronchitis, etc. B-cell
deficiency is the least severe and most common type of IEIs.
Case study
A 3 year old boy shows symptoms including recurrent Otitis media (pus flowing out of the ear),
Pneumonia (3 episodes per year), and sepsis. His immunological evaluation shows normal numbers and
function of phagocytes and T-cells but low titers of immunoglobulins. The almost absent IgG, IgA and
,IgM means that opsonophagocytosis does not work
well, making him more susceptible to bacterial
infections. Cell count shows that not only he doesn't
have antibodies, he also doesn't have B cells.
Upon genetic testing, it was found that there is a
mutation in the BTK gene for upregulating BCR in the
bone marrow (responsiveness to IL7). Without the
gene, there would not be any BCR expressed and
hence no B-cell signaling and halts B-cell development. In every step of the B-cell development cycle,
there could be a genetic problem.
The BTK mutation is X-linked, making the disease called X-linked agammaglobulinemia (XLA). Treatment
to this disease starts with supplementation of gammaglobulines (IgG). If you don't treat the infectious
symptoms, it can lead to recurrent infections, and thus destroy more tissue (scar tissue).
2. Phagocytes
Deficiencies in phagocytes (DC, macrophages) cause impaired clearance of bacteria, fungi and yeast.
Infections occur in airways, blood, and solid organs like the spleen and liver. It is a less severe form of
immunodeficiency.
There could be 3 types of phagocyte diseases:
- Low numbers: granulocytopenia
There are 2 types of granulocytopenia namely congenital neutropenia and autoimmune
neutropenia
- They don’t have the required receptors to process signals (migration defects)
LAD: leukocyte adhesion deficiency (CD11, CD18 deficiency)
- Cells don’t work: dysfunctional phagolysosome
, CGD: chronic granulomatous disease. This disease is characterized by the inability of the
phagocytes to produce ROS that digests the bacteria. The mutations could lie in the NADPH
oxidase complex. CGD is a rare disease but allows invasive infections like Nocardia, Burkholderia
cepacia complex, Staphylococcus aureus, Serratia marcescens, and Aspergillus spp. These
microbes are the most resistant to killing in lysosomes by hydrogen peroxide, hence seen more
often.
3. T-cells
T-cell deficiencies is the most severe type of immunodeficiencies. This leads to the impaired clearance of
viruses, fungi/yeast, and bacteria (T cell dependent B cell function). The patient will have problems with
any type of pathogen in any area of the body. By definition, T-cell disorders are called combined
immunodeficiency as a deficiency of T cells will also impair the B cell activity (T-helper cells). They can
cause all types of infections, severe immune dysregulation, and early onset malignancies.
Case study
A 7 year old boy shows symptoms including recurrent otitis media, pneumonia (3 times per year),
autoimmune hemolytic anemia (breaks down his own RBCs), splenomegaly (giant spleen), varicella
with complicated course (chicken pox), and fungal infections in the nail. His immunological evaluation
shows normal numbers and function of phagocytes but low numbers of T-cells with abnormal
proliferation assays as well as low titers of IgG and IgA (He can still produce some IgM).
He is diagnosed with combined immune deficiency which is a group of diseases with two clinical entities:
- Low T cells and low specific antibodies → infections
treated with antibiotics and immunoglobulins
- Antibodies against self → autoimmune diseases (lower Tregs → activation of auto-T and B cells)
treated with immunosuppressive drugs
Genetic causes identified in 10-20% of patients
of combined immune deficiency. In this case, he
has a homozygous mutation in CTLA-4 which is
proved to be a LOF mutation. CTLA-4 is an
immune checkpoint receptor, responsible for
switching off immune response. Without
CTLA-4, there is no signal 2 to start the inhibitory
cascade leading to T-cell exhaustion, meaning
they get used faster and die more (low total
number), also prolonged inflammation.
Treatments
Antibody deficiencies
Treating antibody deficiencies can be done by immunoglobulin replacement, so we supply the patient
with a donor of IgG (Privigen). The blood from donors are checked for infectious diseases and their IgG
, fraction is pooled. This mixture will have IgG memory from all of the donors providing a broad
protection to the recipient. It can be administered through IV or SC infusion. Ig treatment shows a
positive effect on survival.
Granulocyte deficiencies
Granulocyte diseases can be treated by prophylactic (give in lower dose to patients who are not infected
yet with the pathogen in order to prevent its future infection) antibiotics, antifungal prophylaxis, or a
stem cell transplantation. However, There is no 100% adequate prophylaxis against infections.
Furthermore, stem cell transplantation is only referred to the most severe cases and at high risks of
infections and graft/donor rejection. Gene therapy may solve these issues.
Combined immune deficiency
Since T-cell deficiencies lead to all kinds of infections, it can be treated using antibiotics, virostatic drugs,
and antifungals. However, these drugs will fail soon due to resistance and they are not so good in the
first place. Patients with severe T cell disease need a definitive cure. In theory, Hematopoietic Stem Cell
Transplantation can work as a cure, but in practice is associated with a high percentage of
transplantation related mortality. Gene therapy can be used for known monogenetic cases of IEI.
Gene transfer protocol into autologous bone marrow CD34+ cells
1. Take autologous transduced cells from the patient through bone marrow harvest
2. Purify the hematopoietic stem cells (CD34+)
3. Prestimulation with GF (culture)
4. 3 cycle transduction on retronectin (lentiviral vector containing the ADA corrective gene) +
cytokines
5. Infuse the cells back into the patient. The effect can last quite long (for years) but if there is
rejection, then it has to be done more often.
TREC screening in newborns for earliest detection of SCID
TREC screening is done to screen for T-cell diseases in the
newborn including severe combined immunodeficiency
(SCID). A heel prick is done to collect some blood blots of
the patient. Leftover genes (DNA) from VDJ that are not
used in the TCR are made into a circle called the TREC. It
is not transcribed anymore, only garbage. The presence
of TREC indicates successful VDJ recombination, absence
indicates SCID and non-functional T-cells. After TREC
deficiency is known, more tests will be done to see the
underlying genetic cause.
