A molecular analysis of the cytolytic T lymphocyte response

Liposomes as carriers of antigens and adjuvants

Liposomes have been widely used as carriers of protein or peptide antigens. Antigenic materials can be attached to the outer surface, encapsulated within the internal aqueous spaces, or reconstituted within the lipid bilayers of the liposomes. The natural tendency of liposomes to interact with macrophages has served as the primary rationale for utilizing liposomes as carriers of antigens. Liposomes also serve as carriers of a variety of adjuvants and mediators, including lipid A, muramyl dipeptide and its derivatives, interleukin-1, and interleukin-2. Research utilizing in vitro cell culture models has demonstrated that liposomes containing both appropriate antigens and major histocompatibility gene complex molecules can induce antigen-specific genetically restricted cytotoxic T lymphocytes. Liposomes induce immune reactions through classical interactions with antigen presenting cells. However, modelling experiments have also demonstrated that liposomes can even substitute for antigen presenting cells, and cell-free genetically restricted and nonrestricted presentation of antigens by liposomes to helper T lymphocytes has been demonstrated. Liposomes are successful for inducing potent immunity in vivo and they are now being employed in numerous immunization procedures and as vehicles for candidate vaccines.

Characterization of a factor(s) which synergizes with recombinant interleukin 2 in promoting allogeneic human cytolytic T-lymphocyte responses in vitro

Supernatants from PHA-activated human peripheral blood mononuclear cells, depleted of virtually all IL-2 activity by an anti-rIL-2 immunoadsorbent column, contain a factor(s) which synergizes with rIL-2 in facilitating the generation of allogeneic human CTL responses in vitro. This factor, provisionally termed CTL maturation factor (TcMF), did not appear to promote CTL responses in the absence of rIL-2. Furthermore, it acted later than IL-2 in facilitating CTL responses and could not be replaced by recombinant IFN-gamma. In this report we show that rIFN-alpha, rIL-1 alpha, and rIL-1 beta likewise lack TcMF activity. The TcMF activity in lymphokine-containing culture supernatants could be eliminated by trypsin or pronase but not by neuraminidase or RNase. Gel filtration revealed two peaks of TcMF activity, one at 12,000 to 25,000 Da and the other at 45,000 to 65,000 Da. Isoelectrofocusing demonstrated substantial charge heterogeneity. The majority of TcMF activity was recovered between pI 4.0 and pI 5.5 with a minor component at pI 6.5, corresponding to the areas in which IL-1 activity was also found. However, TcMF activity could be separated from IL-1 by reverse-phase HPLC. Moreover, TcMF recovered following reverse-phase HPLC was also found to be depleted of IL-4 activity. These studies suggest that TcMF activity is mediated by a protein(s) distinct from IL-1, IL-2, IL-4, and interferon-alpha or-gamma.

The isolation and sequence of a novel gene from a human functional T cell line

Using a subtractive hybridization procedure we have constructed a cDNA library enriched for sequences present in functional human T cell lines, but not in human EBV-transformed B cell lines. We have isolated a cDNA clone, AH2-519, representing a novel gene, designated 519. This novel gene is expressed in functional human cytolytic and Th cell lines but not in a variety of other cell lines, including several long-term human T cell tumor lines. The expression of gene 519 is inducible in cultures of normal human PBL using antigenic or mitogenic stimulation. Neither the DNA sequence determined from a full-length cDNA clone overlapping with clone AH2-519 nor the amino acid sequence of its predicted protein product has significant homology to published sequences in the GenBank or NBRF databases. The restricted expression of gene 519 suggests that its gene product is involved in the growth and/or differentiation of normal T cells. The data also show that normal, nontransformed, functional T cells express gene products that can not be readily identified in long-term tumor lines of the same cell lineage.

LFA-1 and other accessory molecules functioning in adhesions of T and B lymphocytes

Adhesions of lymphocytes, among themselves or with other cell types, are necessary for most steps in immune responses including both induction and effector phases. Among adhesions of T cells involving specific immunological recognition, CTL-target adhesions have been the most studied. Although CTL-mediated killing is highly specific (specific/nonspecific lytic activity 50-fold), CTL-target adhesion (conjugation) is less so. In the mouse, specificity of conjugation has typically been four to eightfold. Two recent studies with cloned human CTL found much less specificity of conjugation, from one-fold (no specificity) to 1.5-fold. Thus, with cloned human CTL, adhesion may occur promiscuously with any potential target; recognition following adhesion is necessary for lethal hit delivery. The fact that antibodies to the antigen receptor (Ti or CD3) inhibit killing without inhibiting CTL-target conjugation supports this view. The ability of lymphocytes to form nonspecific adhesions, plus the dependence of even the specific mouse adhesions on temperature, metabolic energy, magnesium, and an intact cytoskeleton suggest that the bulk of the strength of T lymphocyte adhesions are not simply the sum of the bonds between antigen receptors (Ti) and antigen. Lymphocytes evidently possess separate "adhesion strengthening" mechanisms. The similarities in the properties of CTL-target adhesions and antigen-independent homotypic B lymphocyte adhesions (Table 2) suggest that at least some of these mechanisms are widely used among cells of hematopoietic origin. MoAbs to most lymphocyte surface molecules, when bound to the living lymphocyte membrane, have no evident functional effects on lymphocyte function. However, a minority can either activate or inhibit lymphocyte functions. Such antibodies identify "leukocyte (or lymphocyte) function-associated antigens," or LFAs (not all of which happen to have "LFA" in their names, Table 1). Most of the inhibitory antibodies inhibit lymphocyte adhesions, and this appears to account for their inhibitory effects on functions such as killing or proliferation. The fact that the binding of antibodies to a particular membrane glycoprotein inhibits adhesion does not guarantee that the glycoprotein in question is a direct participant in adhesion (one of the "glue" molecules). However, there is scanty evidence in support of indirect "negative signals" that may be induced by such antibodies, and direct participation of most LFAs in adhesion seems likely.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoclonal antibodies against leucoagglutinin-reactive human T lymphocyte surface components. Two antibodies which inhibit cell-mediated cytotoxicity at a post-binding stage

Two out of 20 monoclonal antibodies (IgM, kappa), mAb 3192 and mAb K3G, raised against leucoagglutinin-reactive components on human T cells, effectively blocked lymphocyte-mediated cytotoxicity in vitro. No antigenic polypeptide reactive with these antibodies has been identified thus far. However, they have previously been shown to react specifically with certain neutral glycolipids obtained from spleen. Both mAb inhibited the cytotoxicity of natural killer (NK) cells against K562 cells, antibody-dependent cellular cytotoxicity (ADCC) towards antibody-coated bovine erythrocytes and cytotoxic T lymphocyte activity against allogeneic target cells. In both NK and ADCC, preincubation of the lymphocytes with different antibody concentrations resulted in a dose-dependent reduction of cytotoxicity. In contrast, preincubation of the target cells had no effect indicating that the mAb inhibited cytotoxicity at the effector cell level. When studied at the single-cell level, the mAb did not alter the number of lymphocytes forming conjugates with K562 but significantly reduced the frequency of conjugates containing dead target cells. Addition of the mAb to preformed conjugates resulted in a dose-dependent reduction in the proportion of conjugates containing dead target cells. Furthermore, mAb 3192 did not reduce the number of lymphocytes forming rosettes with bovine erythrocytes, indicating that inhibition of ADCC was not due to blocking of the effector cell-target cell interaction mediated by the Fc receptor of the effector cells. Taken together, these results suggest that the mAb inhibited cytotoxicity by interfering with a post-binding step common for the different cytotoxicity systems.

Alloantigen recognition is preceded by nonspecific adhesion of cytotoxic T cells and target cells

T-cell receptors bind antigens only when the antigens are exposed on the cell surface. This can be studied best in the interaction of cytolytic T lymphocytes (CTL) with target cells because the recognition and binding event can be separated from the lytic phase. Studies with CTL clones specific for HLA-A2 and HLA-B7 demonstrated that conjugates of CTL's and target cells can be formed in the absence of specific antigen recognition. Furthermore, T-cell receptor and target antigen cannot interact unless there is conjugate formation. This indicates that nonspecific conjugate formation between CTL's and target cells precedes the recognition of specific antigen by the T-cell receptor.

Viral-restricted cytolytic T lymphocyte recognition of hybrid human-murine class I histocompatibility antigens

Hybrid human-murine major histocompatibility antigens have been constructed and expressed on the surface of both human RD and murine L cell lines after DNA mediated gene transfer. These antigens linked the polymorphic domains (alpha 1 and alpha 2) of H-2Kb and the carboxy-terminal domains (alpha 3, transmembrane, and intracellular) of HLA-A2. Previously we demonstrated that these antigens were serologically intact and were recognized by allospecific cytolytic T lymphocytes. However, the cell lines expressing the hybrid antigen were less well lysed than the native H-2Kb expressing cell lines. In this study, we extend these observations and demonstrate that virally restricted cytolytic T lymphocytes specific for vesicular stomatitis virus and for Sendai virus can recognize cell lines expressing the hybrid antigen, whether expressed on murine (L cell) or human (RD cell) lines. Furthermore, the data show a profound influence by the carboxy-terminal domains upon the polymorphic T-cell restricting epitopes.

Verapamil inhibition of lymphocyte proliferation and function in vitro

Calcium is a divalent cation which acts both as a cofactor for critical protein function and as a "second signal" to trigger cytosol events after membrane depolarization. The presence of calcium is critical to T-lymphocyte function. Verapamil is a calcium channel blocker at the membrane level and may affect cytosol calcium-calmodulin as well. Verapamil effect was measured on two assays of lymphocyte proliferation (concanavalin A and phytohemagglutinin lectin and alloantigen stimulated) and on cell-mediated cytotoxicity. Verapamil inhibited proliferation at 10(-5) M doses and abrogated it at 10(-4) M. Verapamil interferes with critical calcium-mediated events in T lymphocyte proliferation and function. T-lymphocyte calcium channel blockade may be an effective immunosuppressive strategy.