PKN2 signalling induces stromal cell protrusions to preserve lymph node structural integrity

Abstract

Secondary lymphoid tissues develop specialized stromal networks to facilitate immune cell communication and efficient activation of adaptive immunity. This stromal architecture is robust, maintaining topology throughout extensive remodelling and tissue expansion in response to immune challenge. We have previously reported that cytoskeletal mechanics of the fibroblastic reticular cell (FRC) networks determine tissue tension, and that increased tension initiates stromal proliferation required for lymph node growth. However, it is not known how FRCs mechanistically reshape the stromal network to preserve integrity through the early phases of inflammation. Here, we find a signalling mechanism which coordinates reduced FRC contractility and induction of stromal cell protrusions. RhoA/C GTPase activity is blocked in FRCs to inhibit actomyosin contractility upon contact with CLEC-2⁺ dendritic cells (DCs) and binding to podoplanin⁺ stroma. We now find that an additional Rho GTPase target, the PKC family kinase PKN2, regulates activity of myristoylated alanine-rich protein kinase C-substrate (MARCKS). FRCs use MARCKS to generate protrusions in response to DC contact. We tested this mechanism in vivo and found that PKN2 knock-out lymph nodes are unable to regulate MARCKS and show severely disrupted stromal architecture during lymph node expansion. We now present a mechanism of stromal/immune cell crosstalk which actively induces FRC protrusions – an essential component of lymph node remodelling – to maintain tissue integrity during an adaptive immune response.