Mechanisms of immune defense against viral infections are largely dependent on responses mediated by CD8+ cytotoxic T lymphocytes (CTLs) and NK lymphocytes. Multimers of perforin, a protein contained in cytolytic granules, form pores through which CTL and NK cells release cytotoxic proteins (granzyme B, granulolysin) into virus infected target cells, causing activation of caspases and apoptosis. In resting CTLs and NK lymphocytes, cytotoxic proteins are contained in endosomal secretory lytic granules. Following interaction of CTLs or NK cells with the target cell, vesicles containing lytic granules are transported and dock and fuse with the cell membrane, permitting release of cytotoxic proteins into the target cells.
Hemophagocytic Lymphohistiocytosis
Defects in the intracellular transport and release of cytolytic granules are responsible for various forms of familial HLH, including deficiency of Munc13-4 (required to prime vesicles), syntaxin 11 and Munc18-2 (both of which promote vesicle fusion with the cell mem brane), RHOG and perforin. In these disorders, infections (mostly caused by viruses) trigger an uncontrolled immune response mediated by CD8+ T lymphocytes and NK cells that secrete large amounts of IFN-γ, which activates macrophages. Fever, liver, and spleenic enlargement, lymphadenopathy, profound cytopenias, hypoalbuminemia, coagulopathy, high levels of ferritin and triglycerides, and immune activation (with increased levels of soluble CD25) characterize these acute episodes, which may lead to multiple-organ failure and death. The diagnosis of these familial forms of HLH is facilitated by flow cytometric analysis of surface expression of CD107a (a protein normally contained in endosomal vesicles) upon in vitro activation of cytotoxic T cells and NK cells. Defective expression of CD107a is observed in patients with Munc13-4, Munc18-2, and syntaxin 11 deficiencies. By contrast, this test is normal in patients with perforin deficiency, in which reduced or absent expression of perforin can be easily demonstrated by flow cytometry.
Zinc finger NFX1-type containing 1 (ZNFX1) is an interferon stimulated double-strand RNA sensor. Autosomal recessive ZNFX1 deficiency causes increased susceptibility to viral infections, HLH like manifestations, early-onset seizures, as well as renal and lung disease.
Human Leukocyte Antigen Associated With Pigmentary Dilution Disorders Typical features of HLH are also observed in some pigmentary dilution disorders.149 In particular, Chediak-Higashi syndrome (CHS) is an autosomal recessive disease due to mutations of the LYST gene, which encodes for a protein involved in the sorting of proteins to secretory endosomes. This defect affects not only cytotoxic lymphocytes (accounting for FHL-like clinical features), but also melanocytes (that are unable to transfer melanin to keratinocytes and other epithelial cells) and peripheral neurons. Clinical features of CHS include partial albinism (with silvery hair), progressive peripheral neuropathy, recurrent bacterial infections, and a mild bleeding tendency. Neutropenia is often present and may cause bacterial infections. Giant lysosomes can be identified in circulating leukocytes.
Griscelli syndrome type 2 (GS2) is caused by mutations of the RAB27A gene, which encodes for a protein involved in docking of secretory granules of cytotoxic lymphocytes and melanocytes to the cell membrane. Accordingly, patients with GS2 present with HLH and hypopigmentation. The diagnosis is facilitated by the demonstration of large clumps of pigment distributed irregularly along the hair shaft, and by reduced expression of CD107a on the surface of CD8+ and NK lymphocytes upon in vitro activation.
Hermansky-Pudlak syndrome type 2 (HPS2) and HPS10 are autosomal recessive disorders caused by mutations of the AP3B1 and the AP3D genes, which encodes for the β and the δ components of the AP-3 complex, respectively, involved in sorting of transmembrane proteins to secretory lysosomes. Both disorders are characterized by partial albinism, nystagmus, recurrent bacterial and viral infections, neutropenia and defective cytotoxic activity account for increased risk of HLH. In addition, HPS2 patients have tendency to bleeding due to platelet dysfunction (with absence of dense granules and reduced platelet aggregation), whereas patients with HPS10 have hearing loss and developmental delay.
Irrespective of the nature of the genetic defect, HLH therapy is based on the prompt and aggressive treatment of underlying infections and immunosuppression to curtail macrophage activation. Administration of humanized anti-IFN-γ monoclonal antibodies may block continuous activation of the immune system and induce remission. Ultimately, definitive treatment is based on allogeneic SCT from a matched related or unrelated donor; while survival is better with reduced-intensity conditioning, a high incidence of graft failure favors the use of moderate intensity regimens.153 Mixed chimerism is sufficient to correct the immunologic abnormalities. (See box on Diagnostic and Therapeutic Approach to Cytotoxicity Defects.)
X-Linked Lymphoproliferative Disease
In normal individuals, primary infection with EBV causes infectious mononucleosis, a self-limiting disease. EBV establishes latency in B lymphocytes, salivary glands, and some epithelial cells, and is maintained under control by CD8+ CTLs, and NK lymphocytes. Males with X-linked lymphoproliferative disease type 1 (XLP1) are uniquely susceptible to life-threatening complications of EBV infections.154 XLP1 is caused by mutations of the SH2D1A gene, which encodes for a small adaptor molecule, SLAM-associated protein (SAP). In the absence of SAP, cytotoxic responses to EBV-infected cells are markedly reduced. Persistence of EBV triggers continuous activation of CD8+ CTLs that release high amounts of IFN-γ, ultimately resulting in a macrophage activation syndrome. In addition to HLH, patients with XLP1 are also at high risk of B-cell lymphoma. SAP is also important for the function of follicular helper T cells (TFH ), which promote maturation of antibody responses. Consistent with this, XLP1 males often develop hypogammaglobulinemia with a lack of memory B lymphocytes. Finally, XLP1 is also associated with impaired development of NK T (NKT) lymphocytes. Flow cytometric analysis of SAP expression and mutation analysis at the SH2D1A locus confirms the diagnosis.
A minority of patients with XLP carry defects in another gene (BIRC4) that encodes for the X-linked inhibitor of apoptosis (XIAP). Consistent with this, lymphocytes from patients with this disease (XLP2) show increased susceptibility to activation-induced apoptosis. Compared to XLP1, patients with XLP2 have a higher incidence of HLH (with or without EBV infection), but do have an increased risk of lymphoma. Severe inflammatory bowel disease has been reported in several patients. Hypogammaglobulinemia is often present. Flow cytometric analysis of XIAP expression and mutation analysis at the BIRC4 locus are used to confirm the diagnosis.
Both XLP1 and XLP2 are life-threatening disorders. Administration of immunoglobulins may be beneficial; however, the only curative approach is SCT.
