The role of microtubules in human natural killer cell-mediated cytotoxicity

The mechanobiology of NK cells- 'Forcing NK to Sense' target cells

Natural killer (NK) cells are innate immune lymphocytes that recognize and kill cancer and infected cells, which makes them unique 'off-the-shelf' candidates for a new generation of immunotherapies. Biomechanical forces in homeostasis and pathophysiology accrue additional immune regulation for NK immune responses. Indeed, cellular and tissue biomechanics impact NK receptor clustering, cytoskeleton remodeling, NK transmigration through endothelial cells, nuclear mechanics, and even NK-dendritic cell interaction, offering a plethora of unexplored yet important dynamic regulation for NK immunotherapy. Such events are made more complex by the heterogeneity of human NK cells. A significant question remains on whether and how biochemical and biomechanical cues collaborate for NK cell mechanotransduction, a process whereby mechanical force is sensed, transduced, and translated to downstream mechanical and biochemical signalling. Herein, we review recent advances in understanding how NK cells perceive and mechanotransduce biophysical cues. We focus on how the cellular cytoskeleton crosstalk regulates NK cell function while bearing in mind the heterogeneity of NK cells, the direct and indirect mechanical cues for NK anti-tumor activity, and finally, engineering advances that are of translational relevance to NK cell biology at the systems level.

Distinct roles for the actin nucleators Arp2/3 and hDia1 during NK-mediated cytotoxicity

BACKGROUND

Several actin nucleators, including Arp2/3 and various formins, control numerous cytoskeletal-based functions in vivo.

RESULTS

We investigated the relative roles of these nucleators. As a model system, we used natural killer (NK) lymphocytes, which display a wide range of cytoskeletal-based functions that culminate in the lysis of target cells. NK cells lacking either Arp2/3 or the formin hDia1 were ineffective in target cell lysis, but for distinct reasons. Loss of Arp2/3 function led to defects in cell adhesion and actin assembly at the junction with the target cell (the lytic synapse). In contrast, loss of hDia1 did not disrupt actin assembly at the lytic synapse. Instead, loss of hDia1 led to perturbations in the microtubule cytoskeleton, including the targeting of microtubules to the lytic synapse.

CONCLUSIONS

These studies reveal novel distinctions and relationships among the functions of Arp2/3, formins, and microtubules in cells. Notably, a formin mediates the capture of microtubules at the cell periphery.

The Rac-activating toxin cytotoxic necrotizing factor 1 oversees NK cell-mediated activity by regulating the actin/microtubule interplay

The cell cytoskeleton is widely acknowledged as a master for NK cell function. Specifically, actin filaments guide the NK cell binding to target cells, engendering the formation of the so-called immunological synapse, while microtubules direct the killer behavior. All these cytoskeleton-dependent activities are competently governed by the Rho GTPases, a family of regulatory molecules encompassing the three different subfamilies, Rho, Rac, and Cdc42. By using a Rac GTPase-activating bacterial protein toxin from Escherichia coli named cytotoxic necrotizing factor 1 (CNF1), we obtained results supporting the activation of Rac GTPase as a booster for effector cell-binding efficiency, recruitment ability, and, consequently, cytotoxicity. In particular, the augmented killer capacity of CNF1-treated NK cells was associated with the increased expression of certain cell adhesion or activation-associated molecules and the reshaping of the actin and microtubule networks. Importantly, CNF1 counteracted the activity exerted by toxins disrupting the cytoskeletal architecture. Hence, the activation of Rho GTPases, particularly Rac, induced by CNF1, appears to orchestrate a dynamic cross talk between microtubules and actin filaments, leading to a fruitful NK cell activity and polarization state. Our findings suggest that protein toxins might be viewed as modulators of NK cell cytotoxic activity and could possibly be regarded as useful pharmacological tools for certain Rho-linked immune diseases in the near future.

Roles of Chemokines and Receptor Polarization in NK-Target Cell Interactions

We report that the ability of NK cells to produce chemokines is increased in NK-target cell conjugates. The chemokines produced play a critical role in the polarization and recruitment of NK cells as well as in the NK effector-target cell conjugate formation. Chemokines induce the formation of two specialized regions in the NK cell: the advancing front or leading edge, where chemokine receptors CCR2 and CCR5 cluster, which might guide the cells toward the chemotactic source, and the uropod, where adhesion molecules ICAM-1 and -3 are redistributed. NK cell polarity was intrinsically involved in conjugate formation. The redistribution of both adhesion receptors and CCR was preserved during the formation of NK-target cell conjugates. Time-lapse videomicroscopy studies of the formation of effector-target conjugates showed that morphologic poles are also functionally distinct; while the binding to target cells was preferentially mediated through the leading edge, the uropod was found at the rear of migrating NK cells and recruited additional NK cells to the vicinity of K562 target cells. Inhibition of cell polarization and adhesion receptor redistribution blocked the formation of NK-K562 cell conjugates and the cytotoxic activity of NK cells. We discuss the implication of NK-cell polarization in the development of cytotoxic responses.

Taxol pretreatment of tumor targets amplifies natural killer cell mediated lysis

Taxol is known to polymerize and stabilize microtubules and thereby alter many cellular functions. Our studies examined the effects of taxol pretreatment of tumor targets and cytotoxic effector cells in an effort to determine whether such treatment would result in increased tumor cell lysis without affecting cytotoxic cell function. Our studies demonstrated that taxol concentrations of 6-30 ng/ml which induced approximately 50% growth inhibition and > or = 50% block in the G2/M phase of the K562 cell targets did not have any significant effect on the functional ability of NK cells to lyse K562 cells. Pretreatment of K562 cells with taxol (6 and 30 ng/ml) resulted in an increase in K562 cell lysis by NK cells (or NK cells stimulated with 100 units/ml of rIL-2) in 7 out of 9 donors. The amplification of NK cell-mediated lysis of tumor targets due to taxol pretreatment may provide a combination therapeutic approach which includes taxol treatment followed by rIL-2 stimulation of the immune killer cell function.

Effects of hyperthermia on natural killer cells: inhibition of lytic function and microtubule organization

Cells with natural killer activity (NK) may play an important role in host defence against tumour cells. The lytic function of NK cells is very sensitive to hyperthermic inactivation. However, cells with NK activity isolated from rat spleen and exposed to 41-42.5 degrees C for 30 min could partially recover their cytotoxic activity after incubation at 37 degrees C. The recovered cytotoxicity was still NK-specific, as it only resulted in the lysis of YAC-1 sensitive targets, and could not lyse NK-resistant P815 mastocytoma cells. Conjugate formation assay using NK cells labelled with specific monoclonal antibody (mAb) 3.2.3 indicated that the binding of NK cells to targets was not significantly affected by heat treatment. Compared to controls, however, microtubule organizing centre (MTOC) reorientation towards the region of intercellular contact was reduced by 40% in heated effector cells. This was accompanied by a greater inhibition (62-77%) of NK lytic activity. Kinetic analysis indicated that MTOC reorientation capacity recovered following incubation at 37 degrees C. MTOC recovery was maximal 4 h after treatment whereas that of lytic activity peaked at 6 h. These data indicate that NK cells recover NK-specific lytic activity after heat inactivation. Moreover, our study demonstrates that hyperthermia interferes with post-binding MTOC reorientation, and further supports a role for microtubule in secretory processes involved in NK-mediated cytolysis.