Many pathogens pass through a life-cycle phase, during which they persist or multiply inside host cells. This intracellular lifestyle necessitates the evolution of machineries and mechanisms to enter into host cells but also to exit them. While we have a good understanding of pathogen entry processes, we know much less about the mechanisms of pathogen exit from their host cells. This lack of knowledge is partly due to the view of exit as a passive process. However, most pathogens have evolved specific proteins and mechanisms that allow an orchestrated and temporally defined exit from host cells (Flieger et al., 2018). This special issue contains a collection of research articles, resource articles and reviews that report on the exit mechanisms of bacterial, fungal and parasitic pathogens.

Most of the contributions in this issue come from members of the research network SPP 2225 ‘Exit strategies of intracellular pathogens’ (www.spp2225.rwth-aachen.de) funded as a priority programme of the German Research Foundation DFG. Such priority programmes bring together scientists from across Germany, who work on a new or neglected topic of research with the aim of opening up new research fields. The network fosters collaborations and networking through the organization of different types of conferences or meetings including seminar series and methods workshops for early career researchers, project meetings and scientific symposia during the 6-year funding period. The network also provides funds enabling research stays abroad, start-up projects for early career researchers and training workshops with topics around career planning, scientific communication and gender equality. The network fosters exchange and use of the large variety of techniques established in the different member laboratories and also includes two technical platforms that can be used by SPP 2225 members. Major techniques of the network include combinations of high-end imaging techniques, global analyses based on state-of-the-art transcriptomics, proteomics and lipidomics, genome editing methodologies and proximity-dependent interaction studies in a variety of infection models.

The focus of the SPP 2225 network is the identification of exit pathways used by intracellularly living human pathogens and the functional study of molecules involved in the exit process. Three distinct pathways of host cell exit have been postulated so far, which likely have evolved convergently among the otherwise highly diverse groups of pathogens, that is, the membrane-dependent exit without host cell lysis, the active lytic destruction of the host cell and the initiation of programmed cell death (PCD). Yet, intracellular pathogens clearly utilize more than one of these pathways, dependent on their life-cycle stage, on environmental factors and/or on the type of host cell or tissue. Molecules involved in the exit process are essential for microbial survival and spread and thus also represent important antimicrobial targets.

To date, the best-investigated exit mechanism is host cell lysis through active destruction, a process, during which microbial molecules penetrate or perforate membranes, such as the host cell plasma membrane or the membrane of the vacuolar compartment, and exceptionally can include the walls of intra- or extracellular pathogen-containing cysts. Membrane destruction during host cell exit is particularly known for apicomplexan parasites. In this issue, various articles provide novel insight into molecules involved during host cell exit by Plasmodium and Toxoplasma, like phospholipases (Pietsch et al., 2023), zinc-finger proteins (Farrukh et al., 2023), cytoskeletal elements (Beyeler et al., 2023) or vesicle-resident factors (Sassmannshausen et al., 2023) and plasma membrane-bound proteins (Jennison et al., 2023; Thieleke-Matos et al., 2024). In addition, two reviews summarize apicomplexan exit strategies (Jimenéz-Ruiz et al., 2023; Scheiner et al., 2023). Three more research articles and a review deal with the lytic vacuolar exit by the bacterial pathogens Legionella (Hiller et al., 2023; Neuber et al., 2023), Rhodococcus (Nehls et al., 2024), Salmonella (Scharte et al., 2023), and Staphylococcus (Bayer et al., 2023).

A number of microbial pathogens have been shown to leave the intact host cell by modulating its membrane, while not necessarily destroying it. This membrane-dependent exit includes actin-driven membrane protrusions enabling the spread of single bacteria between cells, extrusions and budding of microbes packed in a membranous compartment as well as ejection, expulsion and exocytosis of free microbes. A study by Gerstenmaier et al. (2023) investigates the machinery of Mycobacterium ejection as an example of membrane-dependent exit.

A third microbial exit strategy is the hijacking of the host cell PCD machinery. PCD is an intrinsic, well-regulated process involving specific signalling cascades and leading to typical morphological changes during cell death as a response to, for example, tissue ontogeny, cellular immunity and the removal of infected cells. PCD encompasses apoptosis, necroptosis, and pyroptosis, but also includes autophagy, and accumulating evidence suggests that the manipulation of PCD-inducing signal transduction pathways represents an exit strategy for a number of intracellular pathogens. In this context, two research articles and a review report on the role of PCD during host cell exit by pathogens like Coxiella (Schulze-Luehrmann et al., 2023), Leishmania (Volkmar et al., 2023) and Candida (Sonnberger et al., 2023).

In addition to the work on the various host cell exit pathways presented in this special issue, a technical report and a resource article illuminate the challenges of imaging bacterial exit (Franzkoch et al., 2023; Schulte et al., 2024). We hope this collection of articles will stimulate the comparatively young field of pathogen exit or egress studies.

许多病原体都会经历生命周期阶段，在此期间它们在宿主细胞内持续存在或繁殖。这种细胞内的生活方式需要机器和机制的进化才能进入宿主细胞，也能离开宿主细胞。虽然我们对病原体进入过程有很好的了解，但我们对病原体从宿主细胞退出的机制知之甚少。这种知识的缺乏部分是由于将退出视为一个被动的过程。然而，大多数病原体已经进化出特定的蛋白质和机制，允许精心策划和临时定义的宿主细胞退出（Flieger et al., 2018 ）。本特刊包含一系列研究文章、资源文章和评论，报告细菌、真菌和寄生虫病原体的退出机制。

本期的大部分贡献来自研究网络 SPP 2225“细胞内病原体的退出策略”(www.spp2225.rwth-aachen.de) 的成员，该网络是德国研究基金会 DFG 的优先项目。此类优先计划汇集了来自德国各地的科学家，他们致力于新的或被忽视的研究主题，旨在开辟新的研究领域。该网络通过在六年资助期内组织不同类型的会议或会议（包括针对早期职业研究人员的研讨会系列和方法讲习班、项目会议和科学研讨会）来促进合作和建立联系。该网络还提供资金支持海外研究、为早期职业研究人员启动项目以及围绕职业规划、科学传播和性别平等主题的培训研讨会。该网络促进不同成员实验室建立的各种技术的交流和使用，还包括两个可供 SPP 2225 成员使用的技术平台。该网络的主要技术包括高端成像技术的组合、基于最先进的转录组学、蛋白质组学和脂质组学的全局分析、基因组编辑方法以及各种感染模型中的邻近依赖性相互作用研究。

SPP 2225 网络的重点是识别细胞内活人类病原体使用的退出途径以及参与退出过程的分子的功能研究。迄今为止，已经假设了三种不同的宿主细胞退出途径，这些途径很可能在高度多样化的病原体群体中趋同进化，即不裂解宿主细胞的膜依赖性退出、宿主细胞的主动裂解性破坏和宿主细胞的主动裂解性破坏。程序性细胞死亡（PCD）的启动。然而，细胞内病原体显然利用这些途径中的一种以上，这取决于它们的生命周期阶段、环境因素和/或宿主细胞或组织的类型。参与退出过程的分子对于微生物的生存和传播至关重要，因此也代表了重要的抗菌靶点。

迄今为止，研究最好的退出机制是通过主动破坏来裂解宿主细胞，在此过程中，微生物分子穿透或穿孔膜，例如宿主细胞质膜或液泡室膜，并且特别可以包括细胞壁细胞内或细胞外含有病原体的包囊。顶复门寄生虫在宿主细胞退出过程中的膜破坏尤其众所周知。在本期中，多篇文章对疟原虫和弓形虫退出宿主细胞过程中涉及的分子提供了新颖的见解，例如磷脂酶（Pietsch 等人， 2023 ）、锌指蛋白（Farrukh 等人， 2023 ）、细胞骨架元件（Beyeler 等人） al.， 2023 ）或囊泡驻留因子（Sassmannshausen 等， 2023 ）和质膜结合蛋白（Jennison 等， 2023 ；Thieleke-Matos 等， 2024 ）。此外，两篇评论总结了 apicomplexan 退出策略（Jimenéz-Ruiz 等人， 2023 ；Scheiner 等人， 2023 ）。另外三篇研究文章和一篇评论涉及细菌病原体军团菌（Hiller 等人， 2023 ；Neuber 等人， 2023 ）、红球菌（Nehls 等人， 2024 ）、沙门氏菌（Scharte 等人，2024）的裂解液泡退出。 ， 2023 ）和葡萄球菌（Bayer 等人， 2023 ）。

许多微生物病原体已被证明可以通过调节宿主细胞的膜而离开完整的宿主细胞，但不一定会破坏它。这种膜依赖性出口包括肌动蛋白驱动的膜突起，使单个细菌能够在细胞之间传播、膜室中微生物的挤出和出芽，以及游离微生物的喷射、排出和胞吐作用。 Gerstenmaier 等人的一项研究。 ( 2023 ) 研究了分枝杆菌排出的机制，作为膜依赖性排出的一个例子。

第三种微生物退出策略是劫持宿主细胞PCD机制。 PCD 是一种内在的、受到良好调节的过程，涉及特定的信号级联，并导致细胞死亡期间典型的形态变化，作为对组织个体发育、细胞免疫和感染细胞去除等的反应。 PCD 包括细胞凋亡、坏死性凋亡和细胞焦亡，还包括自噬，并且越来越多的证据表明，操纵 PCD 诱导的信号转导途径代表了许多细胞内病原体的退出策略。在此背景下，两篇研究文章和一篇综述报告介绍了 PCD 在柯克斯体(Schulze-Luehrmann et al., 2023 )、利什曼原虫(Volkmar et al., 2023 ) 和念珠菌(Sonnberger et al.) 等病原体退出宿主细胞过程中的作用。 ., 2023 ）。

除了本期特刊中介绍的各种宿主细胞退出途径的工作之外，一份技术报告和一篇资源文章阐明了细菌退出成像的挑战（Franzkoch 等人， 2023 年；Schulte 等人， 2024 年）。我们希望这一系列文章能够刺激病原体退出或流出研究这一相对年轻的领域。