Signal transduction through CD4 receptors: stimulatory vs. inhibitory activity is regulated by CD4 proximity to the CD3/T cell receptor.
The binding of antibody to the CD4 molecule inhibits mobilization of cytoplasmic free calcium ([Ca2+]i) in response to CD3 cross-linking on resting T cells. Similarly, when CD3 and CD4 are independently and simultaneously cross-linked, calcium mobilization is inhibited when compared to that induced by cross-linking CD3 alone. In contrast, when anti-CD4 and anti-CD3 are cross-linked together, calcium mobilization is substantially higher than from CD3 cross-linking alone. A heteroconjugate consisting of covalently bound CD3 and CD4 monoclonal antibodies (mAb) retains the ability to mobilize [Ca2+]i in CD4 cells at protein concentrations approximately two orders of magnitude lower than the free CD3 mAb, and the activity of the heteroconjugate is inhibitable by free CD4 mAb. The CD3/CD4 heteroconjugate also shows significantly greater activity in stimulation of inositol phosphate IP1, IP2 and IP3 synthesis in T cells than the CD3 mAb alone, and again the activity is inhibited by free CD4 mAb. The activity of the CD3/CD4 heteroconjugate is not simply due to oligomerization, since CD3/CD3 or CD4/CD4 homoconjugates or homoconjugate mixtures did not show increased activity. Other heteroconjugates (CD3/CD5 and CD3/CD28) were not different than the CD3/CD3 homoconjugate in their ability to increase [Ca2+]i. Purified CD4 T cells that do not respond to CD3 mAb in solution do respond to the CD3/CD4 heteroconjugate in solution by proliferating in the presence of a CD28 mAb, with a significant fraction of CD4 cells entering the second cycle within the first three days of stimulation. The CD3/CD4 heteroconjugate co-modulates the CD3 and CD4 receptors, indicating that the heteroconjugate is not simply anchoring the T cell receptor to the T cell surface like anti-CD3 on a solid surface. These results suggest that CD4 plays an active role in signal transduction when brought into close physical proximity to the CD3/T cell receptor complex during major histocompatibility complex class II-restricted antigen presentation.
Ledbetter
J A
JA
Oncogen, Seattle, WA.
June
C H
CH
Rabinovitch
P S
PS
Grossmann
A
A
Tsu
T T
TT
Imboden
J B
JB
D000911:Antibodies, Monoclonal; D000945:Antigens, Differentiation, T-Lymphocyte / Q000502:physiology*; D017252:CD3 Complex; D002118:Calcium / Q000502:physiology; D003432:Cross-Linking Reagents; D004705:Endocytosis; D006801:Humans; D007158:Immunologic Techniques; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000502:physiology; D008213:Lymphocyte Activation; D046911:Macromolecular Substances; D011948:Receptors, Antigen, T-Cell / Q000502:physiology*; D015728:Receptors, HIV; D011991:Receptors, Virus / Q000502:physiology*; D013601:T-Lymphocytes / Q000502:physiology*
D000911:Antibodies, Monoclonal; D000945:Antigens, Differentiation, T-Lymphocyte; D017252:CD3 Complex; D003432:Cross-Linking Reagents; D007295:Inositol Phosphates; D046911:Macromolecular Substances; D011948:Receptors, Antigen, T-Cell; D015728:Receptors, HIV; D011991:Receptors, Virus; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium