Case 13: X-linked Lymphoproliferative Syndrome

Question 1

how does the immune system defeat viruses?

Answer 1

1) elimination of viruses before they are able to enter host cells

2)
identification and destruction of infected host cells that harbor viruses

both innate and adaptive immune systems are used

Question 2

how does the innate immune system combat viruses?

Answer 2

TLRs sense intracellular
viral particles by identifying prokaryotic nucleic acid fragments within the
host cell

activated TLR induce signaling through interferons regulatory factor (IRF) transcription factors to produce type I interferons = INFα and INF β

INFα and INF β increase lymphocyte retention in the lymph nodes, favoring TH1 differentiation, and
enhancing the cytotoxic effector functions of natural killer (NK) and CD8 T cells

Question 3

how do NK cells fight viruses?

Answer 3

germline DNA receptors so they are

equipped to react immediately upon the
first encounter with the virus and do not require activation, clonal expansion, and
differentiation into effector cells like the cytotoxic T lymphocytes

NK cells express
receptors that activate or inhibit the cell’s killer activity

inhibitory receptors interact with MHCI molecules to prevent NK cells from attacking healthy cells

Question 4

what do the granules of NK cells contain?

Answer 4

perforin

granzymes

these induce apoptosis

IFNγ is also released which activated macrophages and polarized naive T cells into TH1 cells

Question 5

how is a cytotoxic T lymphocyte formed?

Answer 5

a naive virus-specific CD8 T cell is activated to CTL
effector status when its T cell receptor (TCR) recognizes a viral peptide loaded on a
major histocompatibility complex MHCI molecule

CD4 T helper cells are what get activated by MHCII molecules

Question 6

how do CTL kill cells?

Answer 6

1. release of cytotoxic granules
2. granule-independent manner via the Gas ligand which engages the death receptor Fas on a host cell and triggers apoptosis

Question 7

what are NKT cells?

Answer 7

important in the defense against viral infections

they share features of both NK and T cells:

they have CD56 like NK cells but they also have TCR like T cells

these TCRs do not recognize peptides; instead, they identify lipid moieties that are
loaded on an MHC-I-like molecule called CD1

Question 8

how are NK cells activated?

Answer 8

1. MHCI on normal cells is recognized by inhibitory receptors that inhibit signals from activating receptors
2. altered or absent mHCI cannot stimulate a negative signal so the NK cell is triggered by signals from activating receptors
3. activated NK cell releases granule contents, inducing apoptosis in the target cell

Question 9

how does the body usually fight EBV?

Answer 9

EBV infects epithelial cells and B cells

its expansion is
usually well controlled by cytotoxic T cells and NK cells

a primary EBV infection
triggers the activation and cell division of B cells infected by the virus

infected B cells express a number of viral antigens that are targets for specific cytotoxic
responses by NK cells and CD8 T cells that keep the proliferation of infected B cells
under control.

usually EBV is asymptomatic and subsides within a few weeks – after acute infection is resolved the virus persists in a latent form in B cells, salivary glands,
and epithelial cells of the nose and throat and can be shed in saliva

if the virus is reactivated, memory cytotoxic T cells will take care of it

so if you have a T cell deficiency, this is usually liked with increased susceptibility to EBV

Question 10

what are the characteristics of x-linked lymphoproliferative syndrome?

Answer 10

the clinical triad of vulnerability to EBV infections, lymphoma,
and dysgammaglobulinemia

Question 11

what causes XLP?

Answer 11

in 60-70% of cases, XLP is caused by loss
of function of the signaling lymphocytic activation molecule (SLAM)-associated
protein (SAP), which is encoded by the SH2D1A gene on the X chromosome

Question 12

what is SAP?

Answer 12

a cytosolic protein expressed in T, NKT, and NK cells

all these cells are important for clearing viral infections!

SAP protein acts primarily as an adaptor molecule
for the SLAM family of receptors, and mediates SLAM signaling

Question 13

how are SAP and SLAM related?

Answer 13

SAP has an SH2 domain that connects to other proteins

SLAM receptors have an extracellular Ig-like domain and a cytoplasmic tail with ITSM that acts a docking site for SAP

so SAP binds to SLAM; specifically the SH2 domain of SAP binds to ITSM of SLAM at 3 contact sites

Question 14

what happens when SAP binds to SLAM?

Answer 14

a conformational
change in SAP allows it to recruit FynT

FynT then mediates intracellular
signaling and activation of T, NKT, and NK cells

SAP also blocks inhibitory signals to SLAM from other
SH2 domain-containing protein phosphatases via competitive inhibition

Question 15

what happens when there is loss of function of SAP?

Answer 15

reduced activity of the SLAM family

of receptors, causing pleomorphic effects on the immune system

Question 16

where are SLAM receptors found?

Answer 16

dendritic cells, macrophages, NK cells, and B and T lymphocytes

Question 17

what happens when SLAM receptors are activated on CD8 T cells?

Answer 17

in
CD8 T cells, activation of SLAM receptors augments TCR signaling, T cell proliferation,
IFNγ secretion, and cytotoxic granule release

Question 18

how are SAP and SLAM in solved in follicular helper T cells?

Answer 18

TFH
cells express CXCR5 and migrate to the T cell zone of the B cell follicle where they
promote germinal center (GC) formation, isotype switching, and B cell responses
to protein antigens.

SAP-deficiency is thought to interfere with the ability of TFH to
stay in the GC and form stable T-B cell contacts.

Question 19

what is RICD?

Answer 19

RICD = restimulation-induced cell death

SLAM receptors may also regulate excessive T cell activation through RICD

during an exuberant immune response
characterized by excessive pro-inflammatory cytokines, strong T cell stimulation
can result in apoptosis or RICD

RICD acts as a set of breaks on the immune system
to prevent excessive inflammation and lymphoproliferation

Question 20

what happens to NKT cells if there’s loss of SAP function?

Answer 20

SLAM receptor family signaling is also important for the development of premature NKT cells in the thymus

loss of SAP function, and
SLAM activation leads to reduced numbers of NKT cells in XLP patients

Question 21

why are XLP patients vulnerable to EBV?

Answer 21

the defective
cytotoxic responses of CD8 T and NK cells, resulting in the persistence of
EBV‑infected cells

B cells in healthy individuals who are infected
with EBV increase expression of the SLAM family member CD48. CD48 interacts
with another SLAM family member on NK cells, CD244, providing an activating
signal that enables the NK cells to kill the infected B cells

signaling via
CD244 is critical in driving NK killing of EBV-infected target cells. Defects in SLAM
signaling due to the SAP deficiency in XLP prevents this process, allowing EBV to
evade the immune system

Question 22

what is the net effect of XLP?

Answer 22

failure to clear the EBV viruses coupled with defective RICD creates an ongoing
immune stimulus, triggering massive over-proliferation of lymphocytes (lymphoproliferation),
activation of macrophages, and generation of a cytokine storm
or excessive production of inflammatory cytokines such as IFNγ, TNFα, IL-1, IL-2,
and IL-6

overabundance of these cytokines leads to the clinical symptoms
characteristic of fulminant EBV infection that can progress to HLH, including fevers,
cytopenias, and coagulopathy

Question 23

what is XLP-1 vs. XLP-2?

Answer 23

XLP-1 and XLP-2 are used to distinguish

patients with XLP due to SAP or XIAP mutations, respectively

Question 24

what does XIAP do?

Answer 24

1. XIAP inhibits apoptosis by blocking
   the effects of caspases 3, 7, and 9
2. activates the mitogen-activated protein
   kinase and nuclear factor-κB (NFκB) pathways
3. necessary for
   signaling through the nucleotide-binding and oligomerization domain (NOD) 2

Question 25

did Nicholas’s history provide any clues to his diagnosis before his presentation
with fulminant infectious mononucleosis?

Answer 25

in retrospect, the family history and the patient’s past medical history provided clues
that he might be affected by XLP. The patient himself had low IgG and recurrent otitis

his uncle and grandfather were affected by some of the manifestations of XLP,
including recurrent lymphoma and aplastic anemia

Question 26

how can the diagnosis of X-linked lymphoproliferative syndrome be made?

Answer 26

in some patients, family history as well as a classic clinical presentation (fulminant
infectious mononucleosis) provide a strong basis for a clinical diagnosis

often,
however, there is no clear family history and the severe lymphoproliferation cannot
easily be distinguished from other forms of malignant lymphohistiocytosis with
hemophagocytosis.

the molecular defect responsible for most cases of XLP has been
identified as a mutation or deletion of the gene encoding SAP (SH2D1A)

in difficult
cases, a molecular diagnosis is desirable, and PCR analysis of the four exons encoding
SAP can be performed

flow cytometry and Western blotting can be used to identify
patients in whom SH2D1A mutations result in a lack of SAP protein expression

if family history, clinical data, and laboratory data are consistent with a diagnosis of
XLP but no defects in SAP can be identified, mutation analysis should be targeted to
the BIRC4 (XIAP) gene, using PCR

Question 27

if a girl presents with fulminant infectious mononucleosis and evidence of
acute EBV infection, what might be the basis of her disease?

Answer 27

In a girl, mutation of an X-chromosome gene such as SH2D1A would be very unlikely
to account for her problem

an autosomal disorder is more probable

if the parents
were related, suspicion of an autosomal defect would be heightened

autosomal
recessive defects that may cause an XLP-like phenotype include ITK deficiency
but also, and more commonly, defects in the cytolytic machinery that result in
hemophagocytic lymphohistiocytosis

Question 28

nicholas has a 3-year-old brother, Alexander, who has SAP deficiency. what
can be done to prevent the occurrence of disease in this boy?

Answer 28

destruction of Alexander’s bone marrow by radiation or chemotherapy (with busulfan
and cytoxan) followed by administration of HLA-matched marrow from a normal
sibling or unrelated donor would replace all lymphoid precursors with normal
SAP-expressing cells of donor origin

this approach has been used successfully in the
treatment of XLP