Human cytomegalovirus (HCMV) infections of immunocompetent hosts are characterized by a dynamic, life-long interaction in which host immune responses, particularly of T cells, restrain viral replication and prevent disease but do not eliminate the virus or preclude transmission. Because HCMV is among the largest and most complex of known viruses, the T cell resources committed to maintaining this balance have never been characterized completely. Here, using cytokine flow cytometry and 13,687 overlapping 15mer peptides comprising 213 HCMV open reading frames (ORFs), we found that 151 HCMV ORFs were immunogenic for CD4+ and/or CD8+ T cells, and that ORF immunogenicity was influenced only modestly by ORF expression kinetics and function. We further documented that total HCMV-specific T cell responses in seropositive subjects were enormous, comprising on average ∼10% of both the CD4+ and CD8+ memory compartments in blood, whereas cross-reactive recognition of HCMV proteins in seronegative individuals was limited to CD8+ T cells and was rare. These data provide the first glimpse of the total human T cell response to a complex infectious agent and will provide insight into the rules governing immunodominance and cross-reactivity in complex viral infections of humans.

A cytokine was identified that stimulated the proliferation of T lymphocytes, and a complementary DNA clone encoding this new T cell growth factor was isolated. The cytokine, designated interleukin-15 (IL-15), is produced by a wide variety of cells and tissues and shares many biological properties with IL-2. Monoclonal antibodies to the beta chain of the IL-2 receptor inhibited the biological activity of IL-15, and IL-15 competed for binding with IL-2, indicating that IL-15 uses components of the IL-2 receptor.

SummaryInterleukin 7 (IL-7) stimulates the proliferation of B cell progenitors, thymocytes, and mature T cells through an interaction with a high affinity receptor (IL-7R) belonging to the hematopoietin receptor superfamily. We have further addressed the role of IL-7 and its receptor during B and T cell development by generating mice genetically deficient in IL-7R. Mutant mice display a profound reduction in thymic and peripheral lymphoid cellularity. Analyses of lymphoid progenitor populations in IL-7R-deficient mice define precisely those developmental stages affected by the mutation and reveal a critical role for IL-7R during early lymphoid development. Significantly, these studies indicate that the phase of thymocyte expansion occurring before the onset of T cell receptor gene rearrangement is critically dependent upon, and mediated by the high affinity receptor for IL-7.

Human MHC class I-related molecules, MICA and MICB, are stress-induced antigens that are recognized by a subset of ␥␦ T cells expressing the variable region V ␦ 1. This functional association has been found to be limited to intestinal epithelium, where these T cells are prevalent and where MICA and, presumably, MICB are mainly expressed. However, increased frequencies of V ␦ 1 ␥␦ T cells have been observed in various epithelial tumors; moreover, MICA͞B are expressed on diverse cultured epithelial tumor cells. With freshly isolated tumor specimens, expression of MICA͞B was documented in many, but not all, carcinomas of the lung, breast, kidney, ovary, prostate, and colon. In tumors that were positive for MICA͞B, the frequencies of V ␦ 1 ␥␦ T cells were significantly higher than in those that were negative. V ␦ 1 ␥␦ T cell lines and clones derived from different tumors recognized MICA͞B on autologous and heterologous tumor cells. In accord with previous evidence, no constraints were observed in these interactions, such as those imposed by specific peptide ligands. Thus, MICA͞B are tumor-associated antigens that can be recognized, in an apparently unconditional manner, by a subset of tumor-infiltrating ␥␦ T cells. These results raise the possibility that an induced expression of MICA͞B, by conditions that may be related to tumor homeostasis and growth, could play a role in immune responses against tumors.Cytolytic T cell responses against tumors require the recognition of specific peptides derived from tumor antigens in association with MHC class I molecules by CD8 ϩ T cells expressing ␣␤ T cell receptors (TCRs). Such responses are limited by antigenic peptides (1), which must be generated by intracellular antigen processing (2), by their highly selective binding to only some of the numerous polymorphic MHC class I molecules (3), and by the often-impaired expression of MHC class I on tumor cells (4). By contrast, T cells expressing ␥␦ TCRs (5, 6) recognize antigens directly, with no requirement for antigen processing or presentation (7-9). Recently identified ligands for human ␥␦ T cells are MICA and MICB, which are distantly related to MHC class I but are functionally distinct. These molecules seem to have no role in the presentation of intracellular peptide antigens; instead, they may function as self-antigens that can be stress-induced (10). MICA and MICB are closely related and functionally indistinguishable (10-12). Both are recognized by cytotoxic ␥␦ T cells expressing the TCR variable region V ␦ 1. Although the recognition of MICA͞B appears to be mediated by TCR engagement, these interactions are unusual because T cells expressing diverse ␥ and ␦ chain V ␦ 1(D)J junctional sequences are capable of recognizing these molecules (10). This common antigen specificity may enable a subset of V ␦ 1 ␥␦ T cells, which are relatively infrequent and endowed with a limited TCR repertoire, to respond uniformly to expression of MICA͞B as a generic tissue distress signal that may be triggered by infection, noxious c...

SummaryInterleukin 15 (IL-15) is a novel cytokine that has recently been doned and expressed. Whereas it has no sequence homology with IL-2, IL-15 interacts with components of the IL-2 receptor (IL-2R). In the present study we performed a functional analysis of recombinant IL-15 on phenotypically and functionally distinct popttlations of highly purified human natural killer (NK) cells. The CD56 b~ht subset of human NK cells constitutively expresses the high affinity IL-2R and exhibits a brisk proliferative response after the binding of picomolar amounts of IL-2. Using a proliferation assay, IL-15 demonstrated a very steep dose-response curve that was distinct from the dose-response curve for IL-2. The proliferative effects of IL-15 could be abrogated by anti-IL-2R~ (p75), but not by anti-IL-2Rc~ (p55). The proliferative effects of IL-2 on CD56bns ht NK cells could be inhibited by both antibodies. CD56 ~n NK cells express the intermediate affinity IL-2R in the absence of the high affinity IL-2R. Activation of CD56 a~ NK cells by IL-15 was similar to that of IL-2 as measured by enhanced NK cytotoxic activity, antibodydependent cdlular cytotoxicity, and NK cell production of interferon % tumor necrosis factor c~, and granulocyte/macrophage colony-stimulating factor. The IL-15-enhanced NK cytotoxic activity could be completely blocked by anti-IL-21L8 monoclonal antibody. The binding of radiolabded IL-2 and IL-15 to CD56 d~n NK cells was inhibited in the presence of anti-IL-21LB. Scatchard analysis of radiolabeled IL-15 and IL-2 binding to NK-enriched human lymphocytes revealed the presence of high and intermediate affinity receptors for both ligands. IL-15 is a ligand that activates human NK cells through components of the IL-2R in a pattern that is similar but not identical to that of IL-2. Unlike IL-2, IL-15 is produced by activated monocytes/macrophages. The discovery of IL-15 may increase our understanding of how monocytes/macrophages participate in the regulation of NK cell function.

We have recently cloned a novel cytokine, IL‐15, with shared bioactivities but no sequence homology with IL‐2. We found high affinity IL‐15 binding to many cell types, including cells of non‐lymphoid origin. Analysis of IL‐15 interaction with subunits of the IL‐2 receptor (IL‐2R) revealed that the alpha subunit was not involved in IL‐15 binding. We demonstrated directly in cells transfected with IL‐2R subunits that both the beta and gamma chains are required for IL‐15 binding and signaling. Hence, IL‐15, like IL‐2, IL‐4 and IL‐7, utilizes the common IL‐2R gamma subunit found to be defective in X‐linked severe combined immunodeficiency in humans. IL‐15 is the only cytokine other than IL‐2 that has also been shown to share the beta signaling subunit of IL‐2R. The differential ability of some cells to bind and respond to IL‐2 and IL‐15 implies the existence of an additional IL‐15‐specific component.

Two distinct but distantly related complementary DNAs encoding proteins sharing human interleukin-1 (IL-1) activity (termed IL-1 alpha and IL-1 beta), were isolated from a macrophage cDNA library. The primary translation products of the genes are 271 and 269 amino acids long, although expression in Escherichia coli of the carboxy-terminal 159 and 153 amino acids produces IL-1 biological activity.

The CD40 surface molecule is a 277-amino-acid glycoprotein expressed on B lymphocytes, epithelial cells and some carcinoma cell lines. Monoclonal antibodies against CD40 mediate a variety of effects on B lymphocytes, including induction of intercellular adhesion, short- and long-term proliferation, differentiation and enhanced tyrosine phosphorylation of proteins. In addition, germinal centre centrocytes are prevented from undergoing apoptosis by activation through CD40 and receptor for antigen. These data indicate that CD40 could be a receptor for an unknown ligand with important functions in B-cell development and activation. This hypothesis is strengthened by the homology of the extracellular region of the CD40 molecule with a family of cell-surface glycoproteins that includes the receptors for nerve growth factor and tumour necrosis factor. Here we report the cloning of a ligand for CD40 that is expressed on the cell surface of activated T cells and mediates B-cell proliferation in the absence of co-stimulus, as well as IgE production in the presence of interleukin-4.