R – How do restrict access to a class property to only within the same namespace

How do restrict access to a class property to within the same namespace? Consider the following class. The Content class cannot Publish itself, instead the ContentService class
will do a few things before changing the state to published.

public class Content : Entity, IContent
    {
        public string Introduction { get; set; }

        public string Body { get; set; }

        public IList<Comment> Comments { get; set; }

        public IList<Image> Images { get; private set; }

        public State Status { get; } 
    }

public class ContentService
    {
        public IContent Publish(IContent article)
        {
            //Perform some biz rules before publishing   
            article.Status = State.Published;
            return article;
        }
    }

How can i make it so only the ContentService class can change the state of the article?

Are there any deisng patterns to help me deal with this?