Example
ClientContext clientContext = new ClientContext(siteUrl);
SP.List oList = clientContext.Web.Lists.GetByTitle("MyList");
int itemId = 3;
ListItem oListItem = oList.Items.GetById(itemId);
oListItem.BreakRoleInheritance(false);
User oUser = clientContext.Web.SiteUsers.GetByLoginName(@"DOMAIN\alias");
RoleDefinitionBindingCollection collRoleDefinitionBinding = new RoleDefinitionBindingCollection(clientContext);
collRoleDefinitionBinding.Add(clientContext.Web.RoleDefinitions.GetByType(RoleType.Reader));
oListItem.RoleAssignments.Add(oUser, collRoleDefinitionBinding);
clientContext.ExecuteQuery();
