Invariant natural killer T cells developing in the human fetus accumulate and mature in the small intestine.

Invariant natural killer T (iNKT) cells are CD1d-restricted immunoregulatory lymphocytes that share characteristics of both the innate and adaptive immune systems. Although it has been reported that iNKT cells are present in the human fetal thymus, it is currently unknown how they distribute, differentiate, and function in fetal peripheral lymphoid and non-lymphoid organs. Here, we show that functional human fetal iNKT cells develop and differentiate in a tissue-specific manner during the second trimester. Fetal iNKT cells accumulated in the small intestine, where they gained a mature phenotype and mounted robust interferon (IFN)-γ responses. In contrast, iNKT cells in the spleen and mesenteric lymph nodes were less frequently detected, less differentiated, mounted poor IFN-γ responses, but proliferated vigorously upon stimulation with α-galactosylceramide. These data demonstrate that fetal iNKT cells can differentiate and acquire potent effector functions in utero before the establishment of the commensal microflora.

NK cells and CD1d-restricted NKT cells respond in different ways with divergent kinetics to IL-2 treatment in primary HIV-1 infection.

Cytokine immunotherapy is being evaluated as adjunct treatment in infectious diseases. The effects on innate and adaptive immunity in vivo are insufficiently known. Here, we investigate whether combination treatment with antiretroviral therapy (ART) and Interleukin-2 (IL-2) of patients with primary HIV-1 infection induces sustained increases in circulating NKT cell and NK cell numbers and effector functions and investigate how changes are coordinated in the two compartments. Patients with primary HIV-1 infection starting ART were analyzed for numbers, phenotype and function of NKT cells, NK cells and dendritic cells (DC) in peripheral blood before, during and after IL-2 treatment. NKT cells expanded during IL-2 treatment as expected from previous studies. However, their response to α-galactosyl ceramide antigen were retained but not boosted. Myeloid DC did not change their numbers or CD1d-expression during treatment. In contrast, the NK cell compartment responded with rapid expansion of the CD56(dim) effector subset and enhanced IFNγ production. Expansions of NKT cells and NK cells retracted back towards baseline values at 12 months after IL-2 treatment ended. In summary, NKT cells and NK cells respond to IL-2 treatment with different kinetics. Effects on cellular function are distinct between the cell types and the effects appear not to be sustained after IL-2 treatment ends. These results improve our understanding of the effects of cytokine immunotherapy on innate cellular immunity in early HIV-1 infection.

The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A.

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2Dd (Dd), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha1/alpha2 domains. Here we have focused on an exposed loop in the N-terminal part of the alpha2 domain, which constitutes a major structural motif that differs between Dd (strong binding to Ly49A) and Db (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in Dd to the corresponding amino acids in Db. The Dd mutant molecule retained the ability to be stabilized by a Dd-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between Dd and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the Dd mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.

MHC class I recognition by NK receptors in the Ly49 family is strongly influenced by the beta 2-microglobulin subunit.

NK cell recognition of targets is strongly affected by MHC class I specific receptors. The recently published structure of the inhibitory receptor Ly49A in complex with H-2Dd revealed two distinct sites of interaction in the crystal. One of these involves the alpha1, alpha2, alpha3, and beta2-microglobulin (beta2m) domains of the MHC class I complex. The data from the structure, together with discrepancies in earlier studies using MHC class I tetramers, prompted us to study the role of the beta2m subunit in MHC class I-Ly49 interactions. Here we provide, to our knowledge, the first direct evidence that residues in the beta2m subunit affect binding of MHC class I molecules to Ly49 receptors. A change from murine beta2m to human beta2m in three different MHC class I molecules, H-2Db, H-2Kb, and H-2Dd, resulted in a loss of binding to the receptors Ly49A and Ly49C. Analysis of the amino acids involved in the binding of Ly49A to H-2Dd in the published crystal structure, and differing between the mouse and the human beta2m, suggests the cluster formed by residues Lys3, Thr4, Thr28, and Gln29, as a potentially important domain for the Ly49A-H-2Dd interaction. Another possibility is that the change of beta2m indirectly affects the conformation of distal parts of the MHC class I molecule, including the alpha1 and alpha2 domains of the heavy chain.

Purified MHC class I molecules inhibit activated NK cells in a cell-free system in vitro.

Natural killer cells have been shown to interact with MHC class I molecules via inhibitory receptors. However, it is not known whether the inhibition induced by MHC class I molecules requires other NK cell-target cell interactions. Thus, we examined whether purified MHC class I molecules alone were able to inhibit NK cell function. Purified H-2K(b) and H-2D(b) molecules inhibited the release of IFN-gamma from spleen (H-2(b))-derived lymphokine-activated killer (LAK) cell cultures stimulated by anti-NK1.1 antibody in a concentration-dependent manner. LAK cells generated from newborn mice that express low levels of MHC class I binding Ly49 inhibitory receptors were significantly less sensitive to inhibition by H-2K(b) compared to LAK cells from adult mice. Furthermore, LAK cells generated from spleen cells of Ly49C-transgenic mice were significantly more sensitive to inhibition by H-2K(b) compared to non-transgenic littermates. Taken together, the data indicate that MHC class I induced inhibition of NK cell mediated effector functions, as assessed by IFN-gamma release after NK1.1 triggering, does not require additional cell surface molecules other than MHC class I.

Emergence of CD8+ T cells expressing NK cell receptors in influenza A virus-infected mice.

Both innate and adaptive immune responses play an important role in the recovery of the host from viral infections. In the present report, a subset of cells coexpressing CD8 and NKR-P1C (NK1.1) was found in the lungs of mice infected with influenza A virus. These cells were detected at low numbers in the lungs of uninfected mice, but represented up to 10% of the total CD8(+) T cell population at day 10 postinfection. Almost all of the CD8(+)NK1.1(+) cells were CD8alphabeta(+)CD3(+)TCRalphabeta(+) and a proportion of these cells also expressed the NK cell-associated Ly49 receptors. Interestingly, up to 30% of these cells were virus-specific T cells as determined by MHC class I tetramer staining and by intracellular staining of IFN-gamma after viral peptide stimulation. Moreover, these cells were distinct from conventional NKT cells as they were also found at increased numbers in influenza-infected CD1(-/-) mice. These results demonstrate that a significant proportion of CD8(+) T cells acquire NK1.1 and other NK cell-associated molecules, and suggests that these receptors may possibly regulate CD8(+) T cell effector functions during viral infection.

T cell tolerance based on avidity thresholds rather than complete deletion allows maintenance of maximal repertoire diversity.

Given the flexible nature of TCR specificity, deletion or permanent disabling of all T cells with the capacity to recognize self peptides would severely limit the diversity of the repertoire and the capacity to recognize foreign Ags. To address this, we have investigated the patterns of CD8+ CTL reactivity to a naturally H-2Kb-presented self peptide derived from the elongation factor 1alpha (EF1alpha). EF1alpha occurs as two differentially expressed isoforms differing at one position of the relevant peptide. Low avidity CTLs could be raised against both variants of the EF1alpha peptide. These CTLs required 100-fold more peptide-H-2Kb complexes on the target cell compared with CTLs against a viral peptide, and did not recognize the naturally expressed levels of EF1alpha peptides. Thus, low avidity T cells specific for these self peptides escape tolerance by deletion, despite expression of both EF1alpha isoforms in dendritic cells known to mediate negative selection in the thymus. The low avidity in CTL recognition of these peptides correlated with low TCR affinity. However, self peptide-specific CTLs expressed elevated levels of CD8. Furthermore, CTLs generated against altered self peptide variants displayed intermediate avidity, indicating cross-reactivity in induction of tolerance. We interpret these data, together with results previously published by others, in an avidity pit model based on avidity thresholds for maintenance of both maximal diversity and optimal self tolerance in the CD8+ T cell repertoire.

Visualization of inhibitory Ly49 receptor specificity with soluble major histocompatibility complex class I tetramers.

Murine natural killer (NK) cells are inhibited from killing their targets by the interaction between inhibitory, C-type lectin like Ly49 receptors and major histocompatibility complex (MHC) class I molecules. The receptors have overlapping specificity, and it has been difficult to analyze specific aspects of the interaction between different Ly49 receptors and their respective ligands. We have addressed this problem using tetramers of bacterially expressed, non-glycosylated, MHC class I molecules refolded with different peptides. Our results indicate that this technology is useful for analysis of Ly49 receptor specificity as well as for monitoring of NK cell subsets, with the following major conclusions emerging from this study: (1) tetramers of H-2D(d) bound the Ly49A receptor; the MHC associated glycan, previously suggested to be involved in recognition by this receptor, is thus not required for Ly49A receptor binding; (2) in support and extension of a recent report indicating peptide selectivity in the recognition of H-2K(b) by Ly49C(+) cells, H-2K(b) tetramer binding to Ly49C receptors was strongly influenced by the peptide presented by the MHC class I molecule; (3) tetramer binding allowed visualization of interactions that have not previously been detected in functional studies, such as the recognition of H-2D(b) by Ly49A and Ly49C.