Distinct integrin-dependent signals define requirements for lytic granule convergence and polarization in natural killer cells

Abstract

Lytic granules in natural killer (NK) cells represent a dangerous cargo that is targeted for secretion to destroy diseased cells. The appropriate management of these organelles enables the mounting of a precise and valuable host defense. The process of NK cell adhesion to a target cell through engagement of the integrin LFA-1 (lymphocyte function-associated antigen 1) promotes lytic granule organization through complex cellular mechanics and a signaling pathway characterized by Zhang et al. in this issue of Science Signaling. A set of signaling molecules was defined for their ability to promote the polarization of NK cell lytic granules and the microtubule organizing center (MTOC) toward the interface with a target cell. A subset of these signaling molecules was also required for the convergence of lytic granules on the MTOC.

Fig. 1. Signaling requirements for LFA-1–dependent granule convergence and polarization in NK cell cytotoxicity

(A) After engagement of LFA-1, lytic granules in NK cells converge on the MTOC through the minus-end–directed function of the dynein motor complex and signaling by Src family kinases (4, 7). This process can also be activated by stimulating NK cells with interleukin-2 (IL-2) (7). Zhang et al. demonstrated that the activation of ILK, leupaxin, Pyk2, and CLIP-170 in response to LFA-1 engagement was specifically required for the convergence of NK cell lytic granules on the MTOC. Although other NK cell–activating receptors promote convergence, the unbiased subtractive proteomics performed by Zhang et al. has defined these signals as at least minimally required by LFA-1 in inducing convergence function, which thus distinguishes them from those involved in the general activation of NK cells by CD16. ICAM-1, intercellular adhesion molecule–1. (B) Engagement of LFA-1 also stimulates the polarization of lytic granules and the MTOC to the immunological synapse in NK cells. Polarization of the MTOC is likely to depend on synaptic dynein through microtubule end-capture shrinkage (10). Zhang et al. determined that a limited set of signals partially overlapping with those involved in convergence are necessary downstream of LFA-1 to promote polarization. Again, the subtractive proteomics eliminated signals common to general NK cell activation by CD16. For granule polarization to the immunological synapse, LFA-1 engagement caused the recruitment of ILK into close proximity with the cytoplasmic tail of the β₂ subunit of LFA-1, which further activated the downstream signaling molecules γ-parvin, Leupaxin, RhoGEF7, and Pyk2. RhoGEF7, a direct activator of Cdc42, and Pyk2, an indirect activator of Cdc42 (through inhibition of RhoGAP proteins), then stimulate activation of the Cdc42 signaling pathway, which establishes cell polarity during cell migration and division (12). Within the Cdc42 signaling pathway, Par6, APC, and CLIP-170 are required for the polarization of granules and the MTOC in human NK cells. PI3K, phosphatidylinositol 3-kinase.

Fig. 1. Signaling requirements for LFA-1–dependent granule convergence and polarization in NK cell cytotoxicity

(A) After engagement of LFA-1, lytic granules in NK cells converge on the MTOC through the minus-end–directed function of the dynein motor complex and signaling by Src family kinases (4, 7). This process can also be activated by stimulating NK cells with interleukin-2 (IL-2) (7). Zhang et al. demonstrated that the activation of ILK, leupaxin, Pyk2, and CLIP-170 in response to LFA-1 engagement was specifically required for the convergence of NK cell lytic granules on the MTOC. Although other NK cell–activating receptors promote convergence, the unbiased subtractive proteomics performed by Zhang et al. has defined these signals as at least minimally required by LFA-1 in inducing convergence function, which thus distinguishes them from those involved in the general activation of NK cells by CD16. ICAM-1, intercellular adhesion molecule–1. (B) Engagement of LFA-1 also stimulates the polarization of lytic granules and the MTOC to the immunological synapse in NK cells. Polarization of the MTOC is likely to depend on synaptic dynein through microtubule end-capture shrinkage (10). Zhang et al. determined that a limited set of signals partially overlapping with those involved in convergence are necessary downstream of LFA-1 to promote polarization. Again, the subtractive proteomics eliminated signals common to general NK cell activation by CD16. For granule polarization to the immunological synapse, LFA-1 engagement caused the recruitment of ILK into close proximity with the cytoplasmic tail of the β₂ subunit of LFA-1, which further activated the downstream signaling molecules γ-parvin, Leupaxin, RhoGEF7, and Pyk2. RhoGEF7, a direct activator of Cdc42, and Pyk2, an indirect activator of Cdc42 (through inhibition of RhoGAP proteins), then stimulate activation of the Cdc42 signaling pathway, which establishes cell polarity during cell migration and division (12). Within the Cdc42 signaling pathway, Par6, APC, and CLIP-170 are required for the polarization of granules and the MTOC in human NK cells. PI3K, phosphatidylinositol 3-kinase.