Caspases (cysteine aspartate-specific proteases) are a structurally related group of cysteine proteases that cleave peptide bonds following specific recognition sequences. They play a central role in activating apoptosis of vertebrate cells. To measure apoptosis induced by various stimuli and at an early apoptotic stage, caspases are an ideal target. This is especially the case when apoptotic cells have to be analyzed ex vivo before phagocytes remove them. A new and sensitive caspase assay is based on a substrate that contains only aspartate residues linked to rhodamine 110. With this and similar substrates, we are able to detect intracellular caspase activation by flow cytometry after apoptosis induction in intact hematopoetic cell lines.

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The composition of the T lymphocyte population in swine is special in that in addition to the CD4-CD8+ subpopulations, CD4-CD8- and CD4+CD8- and CD4+CD8+ subpopulations are prominent in the peripheral circulating as well as in the resident T lymphocyte pools. Since the same phenotypes are characteristic of thymic populations, it was asked whether the unusual distribution in swine may result from an emigration of thymic precursor phenotypes to the periphery. This explanation was refuted, as all thymic subpopulations were found to express CD1, albeit with differences in antigen density, whereas all extrathymic subpopulations lack CD1. The cellular distribution of CD2 in swine is without precedent among all species studied. Whereas in sheep and cattle the extrathymic CD4-CD8- subpopulation is known to entirely lack CD2 and to have a low propensity for homing to lymphoid tissues, the CD4-CD8- subpopulation in swine splits into CD2+ and CD2- subsets, both of which do reside in lymphoid tissues. While CD2+CD4-CD8- T lymphocytes are rare in the circulating pool, this subset accumulates in spleen and lymph nodes. This may indicate a role for CD2 in homing. Thus the species swine is immunologically unique, not only because of having CD1-CD2+CD4+CD8+ T lymphocytes in the periphery, but also with regard to subdivision and homing behavior of its CD4-CD8- T lymphocyte subpopulation.

Three types of T cell receptors (TcR) gamma/delta were identified in swine, all of which are disulfide-bonded cell surface heterodimers. The first, a 38/40-kDa molecule, is related to the TcR gamma/delta of sheep and cattle, and defines in swine a subset of CD4-CD8- T lymphocytes that is enriched in the circulating T lymphocyte pool. The other two receptors are private to swine. The second, a 37/40-kDa molecule, is expressed by the major subset of CD2-CD4-CD8- T lymphocytes that is neither enriched in nor excluded from lymphoid tissue. By contrast, the third type of TcR gamma/delta, a 40/46-kDa molecule, is found on CD2+CD4-CD8- T lymphocytes accumulating in lymphoid tissue. Thus, TcR gamma/delta T lymphocyte subsets differing in homing propensity are typified by the expression of distinct molecular forms of the TcR gamma/delta.