Effects on the Immune System 393tion, such as NO synthase (NOS) gene transcription or NOS synthesis, rather than
NOS activity was affected by THC. A structure–activity order of effectiveness in
inhibition was noted for various THC analogues with potency being highest for
∆^8 -THC and decreasing in order for THC>CBD≥11-OH-THC>CBN. Further-
more, THC attenuated the cyclic adenosine monophosphate (cAMP) response in
the macrophage cultures. The investigators concluded that inhibition of NO was
mediated by a process that depended partly on a stereoselective cannabinoid re-
ceptor/cAMP pathway and partly on a nonselective molecular process. Jeon et
al. (1996), using the murine RAW264.7 macrophage cell line, demonstrated that
THC inhibited NOS transcription factors such as nuclear factor (NF)-κB/RelA sug-
gesting a mode by which this cannabinoid affected NO production. Waksman et
al. (1999) reported that CP 55,940 mediated inhibition of inducible NOS (iNOS)
produced by neonatal rat brain cortical microglia in a mode that was linked func-
tionallytotheCB 1 receptor. On the other hand, Stephano et al. (2000) indicated that
the synthetic cannabinoid WIN 55,212-2 had an opposite effect on constitutive NO
and increased its release from human monocytes and vascular tissues through the
CB 1 receptor. They reported also that the endocannabinoid 2-arachidonoylglycerol
(2-AG) stimulated NO release from human monocytes and vascular tissues and
immunocytes of the invertebrateMytilus edulis(Stephano et al. 2000). This effect
was mediated through the CB 1 receptor in human cells and through an apparent
“cannabinoid” receptor in the invertebrate immunocytes. Furthermore, in both
the monocytes and the immunocytes, NO release elicited in response to 2-AG
was blocked by a CB 1 ,butnotbyaCB 2 , antagonist. In contrast, Gross et al.
(2000) reported that inhibition of NO production by WIN 55,212-2, but not palmi-
toylethanolamide, was attenuated significantly by the CB 2 antagonist SR144528.
Their results suggested that inhibition of RAW264.7 macrophage-like cell LPS-
induced iNOS expression by WIN 55,212-2 was mediated by the CB 2 receptor.
Cannabinoids have been shown to alter macrophage functions in addition to
those related to cytokine and NO production. It has been reported that THC affects
macrophage processing of antigens that is necessary for the activation of CD4+T
lymphocytes (McCoy et al. 1995). The T cell response to hen egg lysozyme was
dramatically reduced after pretreatment of a macrophage hybridoma with THC.
In contrast, THC exposure did not alter the capacity of the macrophage hybridoma
to process chicken ovalbumin and augmented their presenting cell function for
apigeoncytochromecresponse. The level of T cell activation with peptides of
lysozyme and cytochromec, which do not require processing, was inhibited only
at the highest concentrations of THC, suggesting that THC mainly affected antigen
processing. Peritoneal macrophages exposed to THC during an antigen pulse
and fixed with paraformaldehyde showed similar effects on the subsequent T cell
responses to lysozyme and cytochromecin the absence of THC, arguing against
a possible influence of THC on the T cells. The investigators concluded that THC
differentially modulated the capacity of macrophages to process antigens that is
necessary for the activation of CD4+T cells. Follow-up studies on effects of THC
on processing of intact lysozyme by macrophages provided evidence for a CB 2
receptor participation (McCoy et al. 1999). These observations were confirmed by
Buckley et al. (2000) using CB 2 receptor knockout mice.