Single Degranulations in NK Cells Can Mediate Target Cell Killing.

NK cells are cytotoxic lymphocytes important in defense against viral infection and cancer. NK cells mediate cytotoxicity predominantly through directed secretion of lytic granules, which are specialized lysosome-related organelles, containing effector molecules such as perforin and granzymes. Although many requirements for lytic granule transport to, and secretion at, the NK cell lytic synapse are known, the minimum number of degranulation events required by an NK cell to kill its target is unknown. We performed high-resolution four-dimensional confocal microscopy of human NK-target cell conjugates to quantify NK cell degranulation (using a degranulation indicator, LAMP-1-pHluorin) as well as target cell death. Despite containing almost 200 granules, we found that an individual NK cell needed only two to four degranulation events, on average, to mediate target cell death. Although NK cells released approximately one-tenth of their total lytic granule reserve upon a single target, they required just over one-hundredth of their total lytic granules to kill a target cell. Importantly, the kinetics of NK cell killing correlated to the size of and the amount of effector molecules contained within lytic granules, as well as the temporal, but not spatial, organization of degranulation events. Thus, our study answers a fundamental question as to how many degranulation events it takes for a human NK cell to kill its target.

Phase I trial of low-dose interleukin 2 therapy in patients with Wiskott-Aldrich syndrome.

BACKGROUND: Low dose IL-2 can restore the function of T and NK cells from Wiskott-Aldrich (WAS) patients. However, the safety of in vivo IL-2 in WAS is unknown. OBJECTIVES: A phase-I study to assess safety of low dose IL-2 in WAS. METHODS: Patients received 5 daily subcutaneous IL-2 injections, every 2months, for three courses. A "3+3" dose escalation method was used. RESULTS: 6 patients received the 0.5millionunits/m2/day dose without serious adverse events. However, 2 of 3 patients receiving the 1millionunits/m2/day dose developed thrombocytopenia requiring platelet transfusions. A statistically significant platelet increase occurred in patients receiving the 0.5millionunits/m2/day dose. A trend toward higher T, B and NK cell numbers and higher T regulatory cell percentages was observed. CONCLUSION: We have identified a safe IL-2 dose for WAS patients. Additional trials are indicated to study the efficacy of this immunostimulant as a therapy for WAS.