Staphylococcus aureus peptidoglycan displayed a marked dose-dependent mitogenic activity for mouse splenocytes and human peripheral blood lymphocytes in vitro, as measured by increased [3H]thymidine incorporation. Similarly it was mitogenic for athymic nude mouse spleen cells, whereas no blastogenic effect was observed in T cell-enriched and B cell-depleted mouse lymphocyte cultures. These data demonstrate that peptidoglycan-responding cells in mouse spleen cell cultures are B lymphocytes. Peptidoglycans (PG), heteropolymers present in cell walls of bacteria, actinomycetes and bluegreen algae (cyanobacteria) (25) have been shown to possess numerous biological activities (14, 24), including immunopotentiating properties (15, 16, 18, 19), reticuloendothelial system stimulation (8), complement activation (1, 13, 22), and immunosuppressive effect (9). Mitogenicity for mouse, rabbit, and human lymphocytes was reported for PG from Nocardia species, Escherichia coli, Bacillus megaterium, and PG-rich cell wall fraction from Listeria monocytogenes (4, 5, 6). All these PG have direct interpeptide bridges typical for gram-negative and gram-positive rods, consisting of D

The mitogenic activity of Staphylococcus aureus peptidoglycan (PG), teichoic acid (TA) and cell wall (CW, a PG-TA complex) for mouse, rat and human lymphocytes was determined by measuring DNA synthesis. At optimal concentrations PG was the most potent mitogen, inducing maximum stimulation of mouse spleen lymphocytes on day 2 of culture, rat spleen lymphocytes on day 3, rat lymph node cells on day 2, human peripheral blood lymphocytes on day 5, and human cord blood lymphocytes on day 4. CW was less mitogenic than PG for mouse splenocytes and human peripheral and cord blood lymphocytes, with maximum stimulation occurring on days 2, 6 and 5, respectively. CW was not mitogenic for rat lymphocytes, and TA was not mitogenic for mouse, rat and human lymphocytes. At high concentrations CW and TA were cytotoxic, as indicated by markedly reduced thymidine uptake and low viability. PG was not toxic to lymphocytes. Our results suggest that TA when bound to PG can at high concentrations mask the mitogenic activity of PG and render CW cytotoxic.

We studied secretion of immunoglobulin (Ig) by freshly isolated and pokeweed mitogen (PWM)-stimulated thymus cells and blood mononuclear cells in patients with myasthenia gravis (MG) and control subjects undergoing elective cardiac surgery. We used a protein A reverse hemolytic plaque assay to enumerate cells secreting IgG, IgM, and IgA (IgSC), and an ELISA assay for measuring IgG secreted into culture supernatants. We found that freshly isolated suspensions of MG thymus cells, compared with control thymus cells, contained increased numbers of cells that spontaneously secreted immunoglobulin. Thymus mononuclear cells from control as well as MG patients appeared capable of B-cell differentiation responses when stimulated by PWM. PWM-induced responses were greater in thymic than in autologous blood mononuclear cells in some MG patients and controls, although B cells were much less frequent in suspensions of thymic cells than blood cells. Thus, the thymus provides a favorable milieu for differentiation of its few B cells. In MG, the thymus may be a site of accentuated in vivo B-cell activation, as evidenced by increased numbers of resident IgSC.

Staphylococcal cell wall products have been widely examined as probes for dissection of in vitro human immune responses. Mitogenic and polyclonal B-cellactivating properties have been attributed to intact cell walls or the protein A constituent thereof. We now report that staphylococcal peptidoglycan (PG), the major cell wall constituent, is not only a potent mitogen but also a polyclonal Bcell activator for human peripheral blood mononuclear cells (PBM). PG-induced proliferative responses of human PBM were comparable to that observed in pokeweed mitogen-stimulated cultures. As was true for pokeweed mitogen, PGinduced proliferation required the presence of T-cell help. Cultures of human PBM with PG also resulted in B-cell differentiation as reflected by an increase in numbers of immunoglobulin-secreting cells in stimulated cultures. In contrast to the proliferative response, PG-induced B-cell differentiation was relatively T-cell independent. This point became apparent when B-cell fractions were partially depleted of excessive numbers of monocytes before culture. Also, B-cell proliferation did not appear to be a major prerequisite for PG-induced B-cell differentiation responses. These data indicate that PG is a potent T-cell-dependent mitogen and relatively T-cell-independent polyclonal B-cell activator of human lymphocytes.