Prior studies by our laboratory demonstrated that a single injection of mor
phine produces dose-dependent, naltrexone-reversible, suppressive effects i
n assays of mitogen-stimulated lymphocyte proliferation and natural killer
(NK) cell cytotoxicity in the spleen. The present study used how cytometry
to assess directly whether acute morphine treatment produces these immune a
lterations by altering the leukocyte composition of the spleen. In agreemen
t with our previous findings, morphine suppressed the concanavalin A-stimul
ated proliferation of T cells, lipopolysaccharide-stimulated proliferation
of B cells, and NK cell cytotoxicity in the spleen. However, the same morph
ine treatment protocol did not alter the total number of splenic leukocytes
, the percentage of live splenic leukocytes (as assessed by forward-scatter
versus side-scatter histograms), or the relative number of CD4(+)CD3(+) T
cells, CD8(+)CD3(+) T cells, CD45RA/B+ B cells, NKR-P1A(hi)CD3(-) NK cells,
NKR-P1A(lo)CD3(+) T cells, CD11b/c(+)HIS48(-) monocytes/macrophages, or CD
11b/c(+)HIS48(+) granulocytes in the spleen. These findings indicate that t
he effects of a single sc dose of morphine on functional measures of immune
status in the spleen do not result from a redistribution of splenic leukoc
ytes; instead, morphine's effects likely result from direct alterations in
leukocyte activities. (C) 1999 Academic Press.