In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Depression had a significant negative effect on CD4 + T - cell counts over time regardless of ART use .
### Output Text:
Depression had a significant negative effect on CD4 + T - cell counts over time regardless of ART use . 