T helper and T suppressor cell reversal during the development of induced rat periapical lesions.

T helper (TH) and T suppressor (TS) cells were enumerated in developing rat periapical lesions in order to identify the cell type associated with the active phase of lesion formation. Experimental pulp exposures were made in all molar teeth of 29 Sprague-Dawley rats, and teeth were left open to the oral environment. On days 15, 20, 30, and 90 after induction, inflammatory cells were isolated from periapical lesions, and T helper (W3/25+), T suppressor (OX8+), and total T cells (W3/13+) were enumerated by reactivity with monoclonal antibodies. During the early, most active phase of lesion development (day 15), TH cells outnumbered TS cells (TH/TS = 1.7). However, by day 20, when lesion expansion had slowed, the TH/TS ratio was reversed (0.9) and remained so for up to 90 days after induction (TH/TS = 0.7). Total T cell numbers did not change over the period of observation. TH cells thus predominated during the active phase of lesion development, whereas TS cells were associated with the chronic phase. This dynamic TH/TS reversal indicates that immune responses which occur within periapical lesions are highly regulated. In addition, the predominance of TH during the active phase of lesion development suggests that TH-mediated activities may be involved in the bone destruction and immune cell activation within periapical lesions.