Many human proteins have the potential to bedeveloped as therapeutic agents. However, side effects causedby direct administration of natural proteins have significantlyslowed expansion of protein therapeutics into the clinic. Post-translational modifications (PTMs) can improve proteinproperties, but because of significant knowledge gaps, we areconsiderably limited in our ability to apply PTMs to generatebetter protein therapeutics. Here, we seek to fill the gaps bystudying the PTMs of a small representative chemotacticcytokine, RANTES. RANTES can inhibit HIV-1 infection bycompeting with it for binding to receptor CCR5 and stimulating CCR5 endocytosis. Unfortunately, RANTES can induce strongsignaling, leading to severe inflammatory side effects. We apply a chemical biology approach to explore the potential of post-translationally modified RANTES as safe inhibitors of HIV-1 infection. We synthesized and systematically tested a library ofRANTES isoforms for their ability to inhibit inflammatory signaling and prevent HIV-1 infection of primary human cells.Through this research, we revealed that most of the glycosylated variants have decreased inflammation-associated properties andidentified one particular glyco variant, a truncated RANTES containing a Galβ1−3GalNAc disaccharide α-linked to Ser4, whichstands out as having the best overall properties: relatively high HIV-1 inhibition potency but also weak inflammatory properties.Moreover, our results provided a structural basis for the observed changes in the properties of RANTES. Taken together, thiswork highlights the potential importance of glycosylation as an alternative strategy for developing CCR5 inhibitors to treat HIV-1infection and, more generally, for reducing or eliminating unwanted properties of therapeutic proteins.