Posttranslational processing of chemokines increases (IL-8) or decreases (monocyte chemotactic protein-1) their chemotactic potency. Macrophage-derived chemokine (MDC) attracts monocytes, dendritic cells, activated lymphocytes, and NK cells and has reportedly anti-HIV-1 activity. Here we report that truncation of MDC by deletion of two NH 2-terminal residues resulted in impaired binding to CC chemokine receptor (CCR) 4, the only identified MDC receptor sofar. Truncated MDC (3-69) failed to desensitize calcium mobilization by MDC (1-69) or thymus-and activation-regulated chemokine (TARC), another CCR4 ligand. MDC (3-69) lacked HUT-78 T cell chemotactic activity but retained its capacity to attract monocytes and to desensitize chemotaxis. Compared with MDC (1-69), MDC (3-69) had weak but enhanced antiviral activity against M-and T-tropic HIV-1 strains. Furthermore, both MDC forms failed to signal through the orphan receptors Bonzo/STRL33 and BOB/GPR15 and to desensitize RANTES and stromal cell-derived factor (SDF)-1 responses in CCR5-transfected and CXC chemokine receptor (CXCR) 4-transfected cells, respectively. These findings suggest that MDC recognizes another, yet unidentified, receptor. We conclude that minimal NH 2-terminal truncation of MDC differentially affects its various immunologic functions.