Virtual memory T cells are foreign antigen‐inexperienced T cells that have acquired memory‐like phenotype and constitute 10–20% of all peripheral CD8+ T cells in mice. Their origin, biological roles, and relationship to naïve and foreign antigen‐experienced memory T cells are incompletely understood. By analyzing T‐cell receptor repertoires and using retrogenic monoclonal T‐cell populations, we demonstrate that the virtual memory T‐cell formation is a so far unappreciated cell fate decision checkpoint. We describe two molecular mechanisms driving the formation of virtual memory T cells. First, virtual memory T cells originate exclusively from strongly self‐reactive T cells. Second, the stoichiometry of the CD8 interaction with Lck regulates the size of the virtual memory T‐cell compartment via modulating the self‐reactivity of individual T cells. Although virtual memory T cells descend from the highly self‐reactive clones and acquire a partial memory program, they are not more potent in inducing experimental autoimmune diabetes than naïve T cells. These data underline the importance of the variable level of self‐reactivity in polyclonal T cells for the generation of functional T‐cell diversity.

Mutations in the adaptor protein PSTPIP2 are the cause of the autoinflammatory disease chronic multifocal osteomyelitis in mice. This disease closely resembles the human disorder chronic recurrent multifocal osteomyelitis, characterized by sterile inflammation of the bones and often associated with inflammation in other organs, such as the skin. The most critical process in the disease’s development is the enhanced production of IL-1β. This excessive IL-1β is likely produced by neutrophils. In addition, the increased activity of macrophages, osteoclasts, and megakaryocytes has also been described. However, the molecular mechanism of how PSTPIP2 deficiency results in this phenotype is poorly understood. Part of the PSTPIP2 inhibitory function is mediated by protein tyrosine phosphatases from the proline-, glutamic acid-, serine- and threonine-rich (PEST) family, which are known to interact with the central part of this protein, but other regions of PSTPIP2 not required for PEST-family phosphatase binding were also shown to be indispensable for PSTPIP2 function. In this article, we show that PSTPIP2 binds the inhibitory enzymes Csk and SHIP1. The interaction with SHIP1 is of particular importance because it binds to the critical tyrosine residues at the C terminus of PSTPIP2, which is known to be crucial for its PEST-phosphatase–independent inhibitory effects in different cellular systems. We demonstrate that in neutrophils this region is important for the PSTPIP2-mediated suppression of IL-1β processing and that SHIP1 inhibition results in the enhancement of this processing. We also describe deregulated neutrophil response to multiple activators, including silica, Ab aggregates, and LPS, which is suggestive of a rather generalized hypersensitivity of these cells to various external stimulants.

Overtly self-reactive T cells are removed during thymic selection. However, it has been recently established that T cell self-reactivity promotes protective immune responses. Apparently, the level of self-reactivity of mature T cells must be tightly balanced. Our mathematical model and experimental data show that the dynamic regulation of CD4-and CD8-LCK coupling establish the self-reactivity of the peripheral T cell pool. The stoichiometry of the interaction between CD8 and LCK, but not between CD4 and LCK, substantially increases upon T cell maturation. As a result, peripheral CD8 + T cells are more self-reactive than CD4 + T cells. The different levels of self-reactivity of mature CD8 + and CD4 + T cells likely reflect the unique roles of these subsets in immunity. These results indicate that the evolutionary selection pressure tuned the CD4-LCK and CD8-LCK stoichiometries, as they represent the unique parts of the proximal T cell receptor (TCR) signaling pathway, which differ between CD4 + and CD8 + T cells.

The animal facility of the IMG is a part of the Czech Centre for Phenogenomics and the work there was supported in part by following grants: LM2015040, LM2018126, OP RDI CZ.1.05/2.1.00/19.0395, OP RDI BIOCEV CZ.1.05/1.1.00/02.0109 provided by the Czech Ministry of Education, Youth and Sports and the European Regional Development Fund. AM performed most of the experiments. VN, LS, MP, AD, RS, TM, AN, KK, PS, OS performed experiments. AM, VN, and OS analyzed data and finalized figures. JKu and JM analyzed the transcriptomic data. JN analyzed the TCR profiling data. DC and JKr analyzed the S16 sequencing data. RS and PS provided feral mice. TH and HK provided germ-free mice. MK, HK, JKr, PS, and OS supervised the work. OS conceived the study.

Mature CD8+ T cells use a narrow antigen affinity threshold to generate tissue-infiltrating cytotoxic effector T cells and induce autoimmune pathology, but the mechanisms that establish this antigen affinity threshold are poorly understood. Only antigens with affinities above the threshold induce stable contacts with APCs, polarization of a T cell, and asymmetric T-cell division. Previously published data indicate that LFA-1 inside-out signaling might be involved in establishing the antigen affinity threshold. Here, we show that subthreshold antigens weakly activate all major distal TCR signaling pathways. Lowaffinity antigens are more dependent on LFA-1 than suprathreshold antigens. Moreover, augmenting the inside-out signaling by hyperactive Rap1 does not increase responses to the subthreshold antigens. Thus, LFA-1 signaling does not contribute to the affinitybased antigen discrimination. However, we found that subthreshold antigens do not induce actin rearrangement toward an APC, mediated by Rho-family GTPases, Cdc42, and Rac. Our data suggest that Rac and Cdc42 contribute to the establishment of the antigen affinity threshold in CD8 + T cells by enhancing responses to high-affinity antigens, or by reducing the responses to low-affinity antigens.Keywords: Actin cytoskeleton r Antigen affinity threshold r LFA-1 r Rap1 r Rho-family GTPases r T-cell receptor signalingAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionT cells mediate adaptive immune responses against invading pathogens but must tolerate cells and structures of the host to avoid autoimmunity. The self-tolerant and self-MHC-restricted repertoire of mature T cells is shaped during thymic development by negative and positive selection, respectively. Thymocytes expressing TCRs, recognizing self-peptide-MHC with an affinity Correspondence: Dr. Ondrej Stepanek e-mail: ondrej.stepanek@img.cas.cz above the antigen affinity threshold, are removed by negative selection [1][2][3][4]. CD8/CD4-Lck coupling ratios and a signaling adaptor, Themis, were shown to be critically involved in establishing the affinity threshold for negative selection [4,5]. Peripheral T cells use a narrow antigen affinity threshold to generate differentiated effector T cells and to induce autoimmune pathology, which corresponds to the antigen affinity threshold for negative selection [2,6]. The concordance between the antigen affinity thresholds for negative selection and for effector responses substantially reduces a risk of T-cell-mediated autoimmunity [1,6] OT-I T cells were injected into a RIP-OVA host iv. Mice with glucose concentration in the urine ࣙ5000 mg/dL were considered as diseased. Data shown are derived from n = 3 (OVA, Q4R7) or n = 8 (Q4H7) mice pooled from at least two independent experiments. Statistical significance was calculated using two-tailed Fisher's exact test; OVA versus Q4H7: p = 0.006, Q4R7 versus Q4H7: p = 0.006.T cells, they can trigger some level of proliferation and production of...