Familial hemophagocytic lymphohistiocytosis: when rare diseases shed light on immune system functioning

Abstract

The human immune system depends on the activity of cytotoxic T lymphocytes (CTL), natural killer (NK) cells, and NKT cells in order to fight off a viral infection. Understanding the molecular mechanisms during this process and the role of individual proteins was greatly improved by the study of familial hemophagocytic lymphohistiocytosis (FHL). Since 1999, genetic sequencing is the gold standard to classify patients into different subgroups of FHL. The diagnosis, once based on a clinical constellation of abnormalities, is now strongly supported by the results of a functional flow-cytometry screening, which directs the genetic study. A few additional congenital immune deficiencies can also cause a resembling or even identical clinical picture to FHL. As in many other rare human disorders, the collection and analysis of a relatively large number of cases in registries is crucial to draw a complete picture of the disease. The conduction of prospective therapeutic trials allows investigators to increase the awareness of the disease and to speed up the diagnostic process, but also provides important functional and genetic confirmations. Children with confirmed diagnosis may undergo hematopoietic stem cell transplantation, which is the only cure known to date. Moreover, detailed characterization of these rare patients helped to understand the function of individual proteins within the exocytic machinery of CTL, NK, and NKT cells. Moreover, identification of these genotypes also provides valuable information on variant phenotypes, other than FHL, associated with biallelic and monoallelic mutations in the FHL-related genes. In this review, we describe how detailed characterization of patients with genetic hemophagocytic lymphohistiocytosis has resulted in improvement in knowledge regarding contribution of individual proteins to the functional machinery of cytotoxic T- and NK-cells. The review also details how identification of these genotypes has provided valuable information on variant phenotypes.

Keywords: cellular cytotoxicity; hemophagocytosis; mutation analysis; natural killer.

Figure 1

Overview of the current diagnostic approach to FHL. The clinical suspect is raised by a few elements, which may trigger the use of the functional laboratory screening at flow-cytometry. Mutation analysis remains the gold standard for confirmation of the diagnosis, with therapeutic implications. Notes: 1. Some patients with pigment dilution syndromes (GS, CHS, or HPS) may have gray or very blond hairs, not necessary albino discoloration. 2. Although lack of SAP expression at flow-cytometry is invariably associated with XLP-1, in rare cases patients with XLP-1 may have residual, reduced, or even normal SAP expression; thus the 2B4 assay may be used to discriminate between normal (activating 2B4) or defective (inhibitory 2B4) SAP function. 3. The use in flow-cytometry of currently available anti-XIAP mAb does not provide fully satisfactory results, while it is more suited for western blot analysis. Thus mutation analysis may be necessary to rule out the diagnosis in the presence of suggestive clinical picture.

Figure 2

Familial hemophagocytic lymphohistiocytosis and the immunological synapse. (A) Simplified model of an NK cell synapse formed by a healthy (left) or SAP-deficient (right) cell. In a healthy NK cell, SAP binds to the cross-linked and phosphorylated 2B4 receptor, which leads to the recruitment of Fyn and consequently lysis of the target cell. In the absence of SAP, 2B4 associates with protein tyrosine phosphatases (SHP-1, SHP-2, and SHIP) delivering inhibitory signals. (B) Cartoon of a CTL synapse to illustrate the granule maturation and polarization defect observed in CHS and HPS2 patients, respectively. (C) Magnification of the CTL: target synapse area from (B), depicting the docking defect observed in GS2 patients, the molecular function of Munc13.4 in granule maturation and priming, which is impaired in FHL3 patients, and lack of perforin in the granules of FHL2 patients. (D) Model illustrating the fusion of membranes by the activity of SNARE proteins (such as syntaxin 11) and Munc18-2, which is lost in FHL4 and FHL5 patients.