Exposure to IL-4 during activation of naive murine CD8+ T cells leads to generation of IL-4-producing effector cells with reduced surface CD8, low perforin, granzyme B and granzyme C mRNA, and poor cytolytic function. We show in this study that maximal development of these cells depended on exposure to IL-4 for the first 5 days of activation. Although IL-4 was not required at later times, CD8 T cell clones continued to lose surface CD8 expression with prolonged culture, suggesting commitment to the CD8low phenotype. This state was reversible in early differentiation. When single CD8low cells from 4-day cultures were cultured without IL-4, 65% gave rise to clones that partly or wholly comprised CD8high cells; the proportion of reverted clones was reduced or increased when the cells were cloned in the presence of IL-4 or anti-IL-4 Ab, respectively. CD8 expression positively correlated with perforin and granzyme A, B, and C mRNA, and negatively correlated with IL-4 mRNA levels among these clones. By contrast, most CD8low cells isolated at later time points maintained their phenotype, produced IL-4, and exhibited poor cytolytic function after many weeks in the absence of exogenous IL-4. We conclude that IL-4-dependent down-regulation of CD8 is associated with progressive differentiation and commitment to yield IL-4-producing cells with little cytolytic activity. These data suggest that the CD4−CD8− cells identified in some disease states may be the product of a previously unrecognized pathway of effector differentiation from conventional CD8+ T cells.

Perforin and the serine protease granzymes are key effectors of CD8+ T cell granule-mediated cytotoxicity, but the requirements for their expression remain largely undefined. We show in this study that IL-2 increased the expression of perforin and granzyme A, B, and C mRNA; intracellular granzyme B protein levels; and cytolytic function in a dose-dependent manner during primary activation of murine CD8+ T cells in vitro. Two approaches showed that these responses were not a consequence of the effects of IL-2 on cell survival and proliferation. First, IL-2 enhancement of perforin and granzyme expression was equivalent in CD8+ T cells from wild-type and bcl-2 transgenic mice, although only the latter cells survived in low concentrations or the absence of added IL-2. This property of bcl-2 transgenic T cells also allowed the demonstration that induction of granzyme A, B, and C mRNA and granzyme B protein required exogenous IL-2, whereas induction of perforin and IFN-γ expression did not. Second, analysis of perforin and granzyme mRNA levels in cells separated according to division number using the dye CFSE showed that the effects of IL-2 were unrelated to division number. Together, these findings indicate that IL-2 can directly regulate perforin and granzyme gene expression in CD8+ T cells independently of its effects on cell survival and proliferation.

Here we show that the genes for perforin, the three major T cell granzymes (A-C) and IFN-gamma are differentially expressed during primary activation of naive CD8(+) T cells, kinetically and at the single-cell level. When CD44(low)CD62L(high)CD8(+) lymph node T cells were activated with IL-2 and immobilized antibodies to CD3, CD8 and CD11a, expression of perforin, granzyme B and IFN-gamma mRNAs was induced by day 2, and increased in parallel with perforin-dependent cytolytic activity. Granzyme C and A transcripts were not detected until 1 and 3 days later respectively. Single-cell PCR showed that expression frequencies rose in parallel with total levels of each mRNA, but that individual cells expressed diverse combinations of perforin, granzyme A-C and IFN-gamma mRNAs. These expression patterns indicated that the delayed expression of granzymes A and C was not due to late activation of distinct cell subpopulations. Statistical analysis of the data suggested that each gene was differentially regulated at the single-cell level. Individual naive CD8(+) T cells gave rise over 7 days to clones that expressed all five products at the clonal level, but also expressed diverse combinations at the single-cell level. We conclude that, during primary activation, CD8(+) T cells progressively acquired the ability to express most or all of these genes, and that the variable expression patterns observed among single cells within clones and populations reflected transient rather than heritable differences in expression profile.

Immune deviation of cytolytic T cell function, induced by type 2 cytokines like IL-4, is an attractive concept to explain failure of the immune system in some diseases. However, this concept is challenged by previous conflicting results on whether type 2 cytokine-producing CD8+ T cells are cytolytic. Therefore, we have analyzed the relationship between cytolytic activity and cytokine production among large numbers of primary CD8+ T cell clones. Single murine CD8+ T cells of naive phenotype were activated at high efficiency with immobilized Abs to CD3, CD8, and CD11a in the presence of IL-2 (neutral conditions) or IL-2, IL-4, and anti-IFN-γ Ab (type 2-polarizing conditions) for 8–9 days. Under neutral conditions, most clones produced IFN-γ without IL-4 and were cytolytic. Under type 2-polarizing conditions, most clones produced IFN-γ and IL-4 but displayed variable cytolytic activity and CD8 expression. Separation on the basis of surface CD8 levels revealed that, compared with CD8high cells from the same cultures, CD8low cells were poorly cytolytic and expressed low levels of perforin mRNA and protein and granzyme A, B, and C mRNA. A similar, smaller population of noncytolytic CD8low cells was identified among CD8+ T cells activated in mixed lymphocyte reaction with IL-4. Variable efficiency of generation of the noncytolytic cells may account for the differing results of earlier studies. We conclude that IL-4 promotes the development of a noncytolytic CD8low T cell phenotype that might be important in tumor- or pathogen-induced immune deviation.

The diversity of cytokine production patterns displayed by T cells activated in vivo was investigated by analyzing short-term antigen-specific CD4+ T cell clones and single CD4+ T cells derived from draining lymph nodes of mice undergoing a T helper 2 (Th2)-like response to keyhole limpet hemocyanin (KLH). On average, 2.7% of CD4+ lymph node cells gave rise to clones in the presence of the immunizing antigen and, of these, about 90% secreted interleukin-4 (IL-4) and 20% secreted interferon-gamma (IFN-gamma) when restimulated after 2 weeks in vitro. Almost all IFN-gamma-producing clones co-produced IL-4. The definition of clones as positive or negative for cytokine synthesis depended on assay sensitivity, however, since their titers were distributed continuously from the threshold of detection over at least a 1000-fold range. Reverse-transcription polymerase chain reaction analysis of 59 clones revealed multiple patterns of co-expression of IL-2, IL-3, IL-4, IL-6, IFN-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA. Although most clones contained detectable IL-4 and IL-6 mRNA and a minority contained IFN-gamma mRNA, only 1 clone expressed the canonical Th2 cytokine profile. The observed frequencies of mRNA co-expression for most of the six cytokines (including IL-4 with IFN-gamma), and the frequency of co-secretion of IL-4 and IFN-gamma, were not significantly different from those predicted for random association. Independent regulation of IL-4 and IFN-gamma mRNA expression was confirmed at the single-cell level in a polyclonal population of KLH-primed CD4+ cells, among which co-expression of these cytokines again occurred at the frequency predicted for a random event. The data suggest that the polarization of this immune response towards a Th2 cytokine profile is achieved by altering the probabilities of expression of the IL-4, IFN-gamma and other cytokine genes at the population level, rather than by selective expansion of a distinct T cell subset.