FHL4: NK cells pack less punch

Familial hemophagocytic lymphohistocytosis (HLH) includes several genetic disorders of immune-cell regulation that commonly lead to death if not diagnosed and treated promptly. In this issue of Blood, Bryceson and colleagues provide new insights into the pathogenesis of a subtype of HLH caused by germline mutations in STX11, the gene encoding syntaxin-11.

First described in 1952 by Farquhar and Claireaux as “familial hemophagocytic reticulosis”,¹  familial HLH is now known to comprise a group of rare inflammatory disorders of childhood marked by persistent fever, hepatosplenomegaly, and central nervous system dysfunction. The syndrome is characterized pathologically by the infiltration of affected organs with activated mononuclear cells, phagocytosis of hematopoietic-cell elements by macrophages, and by production of proinflammatory cytokines. Paradoxically, many HLH patients demonstrate depressed cellular immunity with low to absent natural killer (NK)–cell and CD8⁺ T-cell cytotoxicity.

Understanding of the etiology of HLH remained poor until recent years, when several of the genes responsible for this condition were identified. Studies of affected relatives revealed that familial HLH is linked to at least 4 genomic loci, including 9q21, 10q22, 17q25 and 6q24 (known as FHL1–4). The genes within 3 of these loci have been cloned and include PRF1 (FHL2),²  which encodes perforin, a pore-forming protein contained within the lytic granules of T and NK cells, and UNC13D (FHL3)³  and STX11 (FHL4),⁴  which encode Munc13–4 and syntaxin 11, proteins involved in the priming and fusion of perforin-containing granules with the effector-cell plasma membrane (Figure). Although it is not well understood how these gene defects lead to the impaired immune homeostasis that typifies HLH, these findings suggest that impaired T- and NK-cell killing is central to the development of disease, perhaps due to ineffective clearance of infectious agents or perturbed elimination of reactive lymphocytes and antigen-presenting cells following infectious challenge.

The studies reported by Bryceson and coworkers provide further insights into the biology, diagnosis, and management of FHL4. The authors begin by demonstrating that syntaxin 11, a member of the soluble N-ethylmaleimide–sensitive factor-attachment protein receptors present on target membranes (t-SNARE) family, is expressed at readily detectable levels in peripheral-blood T cells and NK cells. When first described as defective in FHL4, syntaxin 11 was found to be expressed in monocytes, not lymphocytes.⁴  Thus, its mechanism of action in HLH remained unclear. By placing syntaxin 11 within T and NK cells, lineages known to be defective in HLH, the authors provide further proof that these cytolytic effectors are central to disease pathogenesis.

If not diagnosed promptly, HLH can be rapidly progressive and fatal. Thus, the therapeutic strategy for these disorders is to treat as soon as a diagnosis has been confirmed, or in some cases strongly suspected. However, establishing a diagnosis can be difficult, as HLH mimics more common illnesses, such as sepsis. Furthermore, it is often challenging to complete traditional immune-cell functional assays due to the limited amount of blood that can be obtained from affected infants, who are often cytopenic as a result of hemophagocytosis. To circumvent these issues, the authors use a flow-cytometry–based method to evaluate the expression of CD107a (LAMP-1), a transmembrane protein present in lytic granules that is exposed at the cell surface following degranulation. They show that, following stimulation, the level of CD107a is increased on a percentage of normal T and NK cells. In contrast, similar stimulation of cells from FHL4 patients led to no detectable CD107a expression, despite proper movement of granules to the lytic synapse. These data are similar to a report by Marcenaro et al of FHL3 patients who harbor UNC13D mutations,⁵  and suggest that syntaxin 11 and Munc 13–4 participate in lytic granule exocytosis after polarization, perhaps at the time of fusion with the plasma membrane.

In primary HLH, the goal of treatment is to control immune-cell activation until a suitable donor for bone marrow transplantation can be identified. Treatment includes steroids to subdue T-cell activation, in conjunction with the chemotherapy agent etoposide, which is toxic to macrophages. Interestingly, the authors find that stimulation of peripheral-blood lymphocytes from FHL4 patients using interleukin 2 or mitogens can partly restore cytotoxicity and enhance degranulation. These findings suggest that alternative mechanisms exist in T and NK cells to support target-cell killing. Such syntaxin-11–independent pathways might compensate for the loss of function of this molecule in FHL4, which could explain the milder clinical phenotype of this HLH subtype. These findings also suggest that medications that “bypass” syntaxin 11 may prove useful as potentially less-toxic treatments for this, and perhaps other, HLH subtypes.

In conclusion, this comprehensive analysis of lymphocytes from adult and pediatric controls, and the side-by-side comparison with cells from FHL patients, provides an important framework for larger studies examining whether CD107a expression can be used in a clinically relevant manner to diagnose primary HLH. Future functional studies of syntaxin 11 may also identify new pathways that might be amenable to pharmacologic manipulation, which could benefit patients with familial HLH as well as other diseases associated with aberrant macrophage activation.