HLH (Hemophagocytic Lymphohistiocytosis)

Blast with Hemophagocytic activity

Fig 1. The Hall mark of HLH is impaired or absent NK cells & Cytotoxic T cell activity. CD4 cytotoxic T cells & NKT (Natural Killer T cells) cytotoxicity is also impaied and contribute to pathogenesis of HLH

Figure (2) shows the sequence of events during CTL (Cytotoxic T Lymphocyte) killing of a cognate target cell. During initial contact an Immunologic Synapse (IS) is formed. MTOC (Micro Tubule Organizing Centre) reorients & polarizes the cytotoxic granule to the IS. At the periphery of Immunologic synapse is the pSMAC (Peripheral Supra Molecular Activation Complex), which is rich in Integrins and it marks the outer boundary of the IS. At the center, is the cSMAC (central SMAC); cSMAC is rich in T cell receptors, and CD8 molecule. The polarized cytotoxic granules fuse with the membrane and form a lytic synapse where released Perforin will perforate the target cell membrane and allow granzyme to enter to induce apoptosis. Only few cytotoxic granules are secreted. The Cytotoxic T cell can bind another target and begin the same process again.

Figure (3) depicts the events occurring at the immunologic synapse and highlights molecules important in process of granule exocytosis.

Steps of granule exocytosis along with proteins required at each step with their defects are given below:

1. Maturation of cytotoxic granule (LYST protein - Disease: Chediak Higashi Syndrome)

2. Granule polarization (Ap3B1 – Disease: Hermansky Pudlak); (SH2D1A; SAP – Disease: X linked Lymphoproliferative Syndrome)

3. Vesicle docking (RAB27α - Griscelli Syndrome)

4. Vesicle priming (UNC13D - FHL3) (Familial HLH)

5. Vesicle Fusion (STX11 - FHL4).

6. Vesicle docking priming & fusion (Munc 18/2 – FHL5)

Defect in Perforin results in FHL2

Figure (4) shows biogenesis and exocytosis of cytotoxic granules.

(SLP1) or SLP2 :synaptotagmin-like protein 1

SNARE: (soluble N-ethylmaleimide-sensitive factor accessory protein receptor)

(VAMP7) and/or VAMP8: vesicle-associated membrane protein 7

SNAP23 (synaptosomal-associated protein of 23 kDa)

Numbers indicate the different steps that are impaired owing to loss of function of the proteins causing inherited haemophagocytic lymphohistiocytosis:

defects in lysosomal trafficking regulator (LYST) (1), adaptor protein 3 (AP3) (2), perforin (3), RAB27a (4), syntaxin 11 (5), MUNC18-2 (6) and MUNC13-4 (7).

Figure (5) shows NK cell & CTL (Cytotoxic T Lymphocyte) response to virally infected cells. In normal patients there is clonal expansion, secretion of Interferon Gamma and killing of virally infected cell resulting in control of viral infection. Once infection is controlled the CTL are culled and immune homeostasis achieved. Some of these cells become memory cells.

Figure (6) shows that in patients of HLH due to genetic defect; there is inability to kill virally infected cell as well as Antigen presenting cells resulting in massive clonal expansion of CTLs, secretion of their cytokines like Interferon Gamma (IFNG), TNF (Tumor Necrosis Factor) alpha, IL6, IL18 & GM-CSF (Granulocyte Macrophage Colony Stimulating Factor). IFNG activate Macrophages, which then phagocytose blood cells resulting in haemophagocytosis. Since the cytokines are produced in a massive amount by Macrophages, T cells & NK cells, hyper stimulation continues and the patient has a cytokine storm with signs & symptoms of HLH. The normal contraction of the immune system also does not take place resulting in persistent cytokine secretion, infiltration of various organs by T cells, massive tissue necrosis & organ failure.

Fig (7) Antigen presentation persist in HLH as the APC (Antigen Presenting Cell) is not eliminated after adequate immune response. This results in persistent stimulation of immune system & enhanced clonal proliferation of CTL, which infiltrate various organs & result in organomegaly, massive tissue necrosis & multiorgan failure.

Fig (8) show an approach to molecular diagnosis of FHL (Familial Hemophagocytic Lymphohistiocytosis.)

Fig (9) highlights an approach to pigmentary dilution and XLP related HLH.

Fig (10) highlights various cell types affected by pigmentary dilution syndrome providing a molecular explanation for various organ system involvement in these disorders. Thus only those disorders (CHS, Griscelli type 2 & Hermansky pudlak syndrome) that affect the NK cells result in HLH. The accelerated phase of CHS is HLH.