The virtuous self-tolerance of virtual memory T cells

FLT3L-induced virtual memory CD8 T cells engage the immune system against tumors

BACKGROUND

Previous research in FMS-like tyrosine kinase 3 ligands (FLT3L) has primarily focused on their potential to generate dendritic cells (DCs) from bone marrow progenitors, with a limited understanding of how these cells affect CD8 T cell function. In this study, we further investigated the in vivo role of FLT3L for the immunomodulatory capabilities of CD8 T cells.

METHODS

Albumin-conjugated FLT3L (Alb-FLT3L) was generated and applied for translational medicine purposes; here it was used to treat naïve C57BL/6 and OT1 mice for CD8 T cell response analysis. Syngeneic B16ova and E.G7ova mouse models were employed for adoptive cell transfer to evaluate the effects of Alb-FLT3L preconditioning of CD8 T cells on tumor progression. To uncover the underlying mechanisms of Alb-FLT3L modulation, we conducted bulk RNA-seq analysis of the CD44high CD8 T cells. STAT1-deficient mice were used to elucidate the functional roles of Alb-FLT3L in the modulation of T cells. Finally, antibody blockade of type one interferon signaling and in vitro coculture of plasmacytoid DCs (pDCs) with naive CD8 T cells was performed to determine the role of pDCs in mediating regulation of CD44high CD8 T cells.

RESULTS

CD44high CD8 T cells were enhanced in C57BL/6 mice administrated with Alb-FLT3L. These CD8 T cells exhibited virtual memory features and had greater proliferative and effective functions. Notably, the adoptive transfer of CD44high naïve CD8 T cells into C57BL/6 mice with B16ova tumors led to significant tumor regression. RNA-seq analysis of the CD44high naïve CD8 T cells revealed FLT3L to induce CD44high CD8 T cells in a JAK-STAT1 signaling pathway-dependent manner, as supported by results indicating a decreased ability of FLT3L to enhance CD8 T cell proliferation in STAT1-deficient mice as compared to wild-type control mice. Moreover, antibody blockade of type one interferon signaling restricted the generation of FLT3L-induced CD44high CD8 T cells, while CD44 expression was able to be induced in naïve CD8 T cells cocultured with pDCs derived from FLT3L-treated mice. This suggests the crucial role of pDCs in mediating FLT3L regulation of CD44high CD8 T cells.

CONCLUSIONS

These findings provide critical insight and support the therapeutic potential of Alb-FLT3L as an immune modulator in preconditioning of naïve CD8 T cells for cancer immunotherapy.

Novel murine model reveals an early role for pertussis toxin in disrupting neonatal immunity to Bordetella pertussis

The increased susceptibility of neonates to specific pathogens has previously been attributed to an underdeveloped immune system. More recent data suggest neonates have effective protection against most pathogens but are particularly susceptible to those that target immune functions specific to neonates. Bordetella pertussis (Bp), the causative agent of "whooping cough", causes more serious disease in infants attributed to its production of pertussis toxin (PTx), although the neonate-specific immune functions it targets remain unknown. Problematically, the rapid development of adult immunity in mice has confounded our ability to study interactions of the neonatal immune system and its components, such as virtual memory T cells which are prominent prior to the maturation of the thymus. Here, we examine the rapid change in susceptibility of young mice and define a period from five- to eight-days-old during which mice are much more susceptible to Bp than mice even a couple days older. These more narrowly defined "neonatal" mice display significantly increased susceptibility to wild type Bp but very rapidly and effectively respond to and control Bp lacking PTx, more rapidly even than adult mice. Thus, PTx efficiently blocks some very effective form(s) of neonatal protective immunity, potentially providing a tool to better understand the neonatal immune system. The rapid clearance of the PTx mutant correlates with the early accumulation of neutrophils and T cells and suggests a role for PTx in disrupting their accumulation. These results demonstrate a striking age-dependent response to Bp, define an early age of extreme susceptibility to Bp, and demonstrate that the neonatal response can be more efficient than the adult response in eliminating bacteria from the lungs, but these neonatal functions are substantially blocked by PTx. This refined definition of "neonatal" mice may be useful in the study of other pathogens that primarily infect neonates, and PTx may prove a particularly valuable tool for probing the poorly understood neonatal immune system.

Shaping Heterogeneity of Naive CD8+ T Cell Pools

Immune diversification helps protect the host against a myriad of pathogens. CD8⁺ T cells are essential adaptive immune cells that inhibit the spread of pathogens by inducing apoptosis in infected host cells, ultimately ensuring complete elimination of infectious pathogens and suppressing disease development. Accordingly, numerous studies have been conducted to elucidate the mechanisms underlying CD8⁺ T cell activation, proliferation, and differentiation into effector and memory cells, and to identify various intrinsic and extrinsic factors regulating these processes. The current knowledge accumulated through these studies has led to a huge breakthrough in understanding the existence of heterogeneity in CD8⁺ T cell populations during immune response and the principles underlying this heterogeneity. As the heterogeneity in effector/memory phases has been extensively reviewed elsewhere, in the current review, we focus on CD8⁺ T cells in a "naïve" state, introducing recent studies dealing with the heterogeneity of naive CD8⁺ T cells and discussing the factors that contribute to such heterogeneity. We also discuss how this heterogeneity contributes to establishing the immense complexity of antigen-specific CD8⁺ T cell response.

Signals for antigen-independent differentiation of memory CD8+ T cells

Conventional CD8+ memory T cells develop upon stimulation with foreign antigen and provide increased protection upon re-challenge. Over the past two decades, new subsets of CD8+ T cells have been identified that acquire memory features independently of antigen exposure. These antigen-inexperienced memory T cells (TAIM) are described under several names including innate memory, virtual memory, and memory phenotype. TAIM cells exhibit characteristics of conventional or true memory cells, including antigen-specific responses. In addition, they show responsiveness to innate stimuli and have been suggested to provide additional levels of protection toward infections and cancer. Here, we discuss the current understanding of TAIM cells, focusing on extrinsic and intrinsic molecular conditions that favor their development, their molecular definitions and immunological properties, as well as their transcriptional and epigenetic regulation.

The histone methyltransferase DOT1L prevents antigen-independent differentiation and safeguards epigenetic identity of CD8+ T cells

Cytotoxic T cell differentiation is guided by epigenome adaptations, but how epigenetic mechanisms control lymphocyte development has not been well defined. Here we show that the histone methyltransferase DOT1L, which marks the nucleosome core on active genes, safeguards normal differentiation of CD8+ T cells. T cell-specific ablation of Dot1L resulted in loss of naïve CD8+ T cells and premature differentiation toward a memory-like state, independent of antigen exposure and in a cell-intrinsic manner. Mechanistically, DOT1L controlled CD8+ T cell differentiation by ensuring normal T cell receptor density and signaling. DOT1L also maintained epigenetic identity, in part by indirectly supporting the repression of developmentally regulated genes. Finally, deletion of Dot1L in T cells resulted in an impaired immune response. Through our study, DOT1L is emerging as a central player in physiology of CD8+ T cells, acting as a barrier to prevent premature differentiation and controlling epigenetic integrity.

Virtual memory CD8+ T cells restrain the viral reservoir in HIV-1-infected patients with antiretroviral therapy through derepressing KIR-mediated inhibition

The viral reservoir is the major hurdle in developing and establishing an HIV cure. Understanding factors affecting the size and decay of this reservoir is crucial for the development of therapeutic strategies. Recent work highlighted that CD8+ T cells are involved in the control of viral replication in ART-treated HIV-1-infected individuals, but how CD8+ T cells sense and restrict the HIV reservoir are not fully understood. Here, we demonstrate that a population of unconventional CD45RA+, PanKIR+, and/or NKG2A+ virtual memory CD8+ T cells (T_VM cells), which confer rapid and robust protective immunity against pathogens, plays an important role in restraining the HIV DNA reservoir in HIV-1-infected patients with effective ART. In patients undergoing ART, T_VM cells negatively correlate with HIV DNA and positively correlate with circulating IFN-α2 and IL-15. Moreover, T_VM cells constitutively express high levels of cytotoxic granule components, including granzyme B, perforin and granulysin, and demonstrate the capability to control HIV replication through both cytolytic and noncytolytic mechanisms. Furthermore, by using an ex vivo system, we showed that HIV reactivation is effectively suppressed by T_VM cells through KIR-mediated recognition. This study suggests that T_VM cells are a promising target to predict posttreatment virological control and to design immune-based interventions to reduce the reservoir size in ART-treated HIV-1-infected individuals.