The search for and analysis of new ligands for innate immunity receptors are of special significance for understanding the regulatory mechanisms of immune response. Here we show that the major heat shock protein 70 (Hsp70) can bind to and activate TREM-1, the innate immunity receptor expressed on monocytes. The Hsp70–TREM-1 interaction activates expression of TNFα and IFNγ mRNAs in monocytes and stimulates IL-2 secretion by PBMCs. Moreover, incubation of PBMCs with Hsp70 leads to an appearance of cytotoxic lymphocyte subpopulations active against the MHC-negative tumor cells. In addition, both the CD4+ T-lymphocytes and CD14+ monocytes are necessary for the Hsp70 signal transduction and a consequent activation of the cytotoxic lymphocytes. We believe that data presented in this study will broaden the views on the involvement of Hsp70 in the antitumor immunity.

Studies on the mechanisms of activation of cytotoxic lymphocyte subpopulations are an important research direction in modern immunology. This study provides a detailed analysis of the effect of Tag7 (PGRP-S, PGLYRP1) on the development of lymphocyte subpopulations cytotoxic against MHC-negative tumor cells in a pool of peripheral blood mononuclear cells (PBMCs). The results show that Tag7 can bind to the TREM-1 receptor on the surfaces of monocytes, thereby triggering the expression of mRNA TNFα and IFNγ. The appearance of these cytokines in conditioned medium leads to IL-2 cytokine secretion by CD3⁺CD4⁺ lymphocytes. In turn, IL-2 facilitates unspecific activation of three cytotoxic cell subpopulations in the PBMC pool: NK (CD16⁺CD56⁺), CD3⁺CD4⁺ and CD3⁺CD8⁺. These subpopulations appear after a certain period of incubation with Tag7 and show toxicity against tumor cells.

Infection caused by the severe acute respiratory syndrome coronavirus (SARS-CoV-2) in many cases is accompanied by the release of a large amount of proinflammatory cytokines in an event known as “cytokine storm”, which is associated with severe coronavirus disease 2019 (COVID-19) cases and high mortality. The excessive production of proinflammatory cytokines is linked, inter alia, to the enhanced activity of receptors capable of recognizing the conservative regions of pathogens and cell debris, namely TLRs, TREM-1 and TNFR1. Here we report that peptides derived from innate immunity protein Tag7 inhibit activation of TREM-1 and TNFR1 receptors during acute inflammation. Peptides from the N-terminal fragment of Tag7 bind only to TREM-1, while peptides from the C-terminal fragment interact solely with TNFR1. Selected peptides are capable of inhibiting the production of proinflammatory cytokines both in peripheral blood mononuclear cells (PBMCs) from healthy donors and in vivo in the mouse model of acute lung injury (ALI) by diffuse alveolar damage (DAD). Treatment with peptides significantly decreases the infiltration of mononuclear cells to lungs in animals with DAD. Our findings suggest that Tag7-derived peptides might be beneficial in terms of the therapy or prevention of acute lung injury, e.g., for treating COVID-19 patients with severe pulmonary lesions.

An investigation of innate immunity receptors sheds light on the mechanisms of inflammation and associated immune reactions. One of the key immune regulators is the TREM-1 receptor, which is involved in both inflammation and antitumor immune response. In this article, we have obtained a new ligand for the TREM-1 receptor. The peptide, named N3, is a part of the innate immune protein PGLYRP1/Tag7. It is responsible for activating the TREM-1 signaling pathway. Here, we have demonstrated that the N3 peptide acts like other TREM-1 receptor ligands: its binding results in a mild inflammation response and appearance of cytotoxic lymphocytes. We have shown that cytotoxic populations of lymphocytes in N3 peptide-treated PBMCs are similar to those treated with Tag7 or Hsp70. We also determined the part of the N3 peptide responsible for binding to TREM-1. The resulting peptide (N9) consists of nine amino acids and can be considered as a potential peptide that blocks TREM-1 signaling.

The discovery of new chemokines that induce the migration of lymphocytes to the infection site is important for the targeted search for therapeutic agents in immunotherapy. We recently showed that Tag7 (PGLYRP1), an innate immunity protein, forms a stable complex with the Ca2+ -binding protein Mts1 (S100A4), which is able to induce lymphocyte movement, although the individual Tag7 and Mts1 do not have this activity. The purpose of this study is to identify receptors that induce the migration of lymphocytes along the concentration gradient of the Tag7-Mts1 complex, and the components of this complex capable of interacting with these receptors. The study investigated the migration of human PBMC under the action of the Tag7-Mts1complex. PBMC of healthy donors were isolated using a standard Ficoll-Hypaque gradient centrifugation procedure. It has been established that the movement of PBMC along the concentration gradient of the Tag7-Mts1 complex is induced by the classical chemotactic receptors CCR5 and CXCR3. It has been shown that only Mts1 is able to bind to the extracellular domain of CCR5, however, this binding is not enough to induce cell movement. A comparative analysis of the primary and 3D structures of the three proteins revealed the homology of the amino acid sequence fragments of the Tag7-Mts1 protein complex with different sites of the CCR5 receptor ligand - MIP1 protein. In conclusion, it should be noted that the Tag7-Mts1 complex can be considered as a new ligand of the classical chemotactic receptors CCR5 and CXCR3.

One of the promising fields of modern molecular biology is the search for new proteins that regulate the various stages of the immune response and the investigation of the molecular mechanisms of action of these proteins. Such proteins include the multifunctional protein PGLYRP1/Tag7, belonging to the PGRP-S protein family, whose gene was discovered in mice at the Institute of Gene Biology, Russian Academy of Sciences, in 1996. PGLYRP1/Tag7 is classified as a protein of innate immunity; however, it can also participate in the regulation of acquired immunity mechanisms. In this paper, we consider the involvement of PGLYRP1/Tag7 in the triggering of antimicrobial defense mechanisms and formation of subsets of cytotoxic lymphocytes that kill tumor cells. The paper emphasizes that the multifaceted functional activity of Tag7 in the immune response has to do with its ability to interact with various proteins to form stable protein complexes. Hsp70-associated Tag7 can induce the death of tumor cells carrying the TNFR1 receptor. Tag7, associated with the Mts1 (S100A4) protein, can stimulate the migration of innate and adaptive immune cytotoxic lymphocytes to a lesion site. Involvement of Tag7 in the regulation of immunological processes suggests that it may be considered as a promising agent in cancer therapy. These properties of Tag7 were used to develop autologous vaccines that have passed the first and second phases of clinical trials in patients with end-stage melanoma and renal cancer. The C-terminal peptide of Tag7, isolated by limited proteolysis, was shown to protect the cartilage and bone tissue of the ankle joint in mice with induced autoimmune arthritis and may be a promising drug for suppressing the development of inflammatory processes.